LKML Archive mirror
 help / color / mirror / Atom feed
From: Jarkko Sakkinen <jarkko@kernel.org>
To: Kai Huang <kai.huang@intel.com>
Cc: kvm@vger.kernel.org, x86@kernel.org, linux-sgx@vger.kernel.org,
	linux-kernel@vger.kernel.org, seanjc@google.com, luto@kernel.org,
	dave.hansen@intel.com, rick.p.edgecombe@intel.com,
	haitao.huang@intel.com, pbonzini@redhat.com, bp@alien8.de,
	tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
	jethro@fortanix.com, b.thiel@posteo.de, jmattson@google.com,
	joro@8bytes.org, vkuznets@redhat.com, wanpengli@tencent.com,
	corbet@lwn.net
Subject: Re: [PATCH v3 00/25] KVM SGX virtualization support
Date: Fri, 19 Mar 2021 16:52:01 +0200	[thread overview]
Message-ID: <YFS6kTe1SuAjiMFN@kernel.org> (raw)
In-Reply-To: <cover.1616136307.git.kai.huang@intel.com>

On Fri, Mar 19, 2021 at 08:29:27PM +1300, Kai Huang wrote:
> This series adds KVM SGX virtualization support. The first 14 patches starting
> with x86/sgx or x86/cpu.. are necessary changes to x86 and SGX core/driver to
> support KVM SGX virtualization, while the rest are patches to KVM subsystem.
> 
> This series is based against latest tip/x86/sgx, which has Jarkko's NUMA
> allocation support.
> 
> You can also get the code from upstream branch of kvm-sgx repo on github:
> 
>         https://github.com/intel/kvm-sgx.git upstream
> 
> It also requires Qemu changes to create VM with SGX support. You can find Qemu
> repo here:
> 
> 	https://github.com/intel/qemu-sgx.git upstream
> 
> Please refer to README.md of above qemu-sgx repo for detail on how to create
> guest with SGX support. At meantime, for your quick reference you can use below
> command to create SGX guest:
> 
> 	#qemu-system-x86_64 -smp 4 -m 2G -drive file=<your_vm_image>,if=virtio \
> 		-cpu host,+sgx_provisionkey \
> 		-sgx-epc id=epc1,memdev=mem1 \
> 		-object memory-backend-epc,id=mem1,size=64M,prealloc
> 
> Please note that the SGX relevant part is:
> 
> 		-cpu host,+sgx_provisionkey \
> 		-sgx-epc id=epc1,memdev=mem1 \
> 		-object memory-backend-epc,id=mem1,size=64M,prealloc
> 
> And you can change other parameters of your qemu command based on your needs.
> 
> =========
> Changelog:
> 
> (Changelog here is for global changes. Please see each patch's changelog for
>  changes made to specific patch.)
> 
> v2->v3:
> 
>  - No big change in design, structure of patch series, etc.
>  - Rebased to lastest tip/x86/sgx, to resolve merge conflict of patch 3
>    (x86/sgx: Wipe out EREMOVE from sgx_free_epc_page()).
>  - Addressed some Nit issues found by Sean in v2.
>  - Also addressed some Nit issues reported by checkpatch.pl. Now there's no
>    checkpatch issues.
>  - Updated patch 3 (x86/sgx: Wipe out EREMOVE from sgx_free_epc_page()):
>    - Removed Jarkko from author, per request.
>    - Changed to replace all call sites of sgx_free_epc_page() with new
>      sgx_encl_free_epc_page(), to make this patch doesn't have functional
>      changes (except a WARN upon EREMOVE failure requestd by Dave).
>    - Rebased to tip/x86/sgx, which has Jarkko's NUMA allocation.
>    - Added Jarkko's Acked-by.
>  - Updated patch 8 (x86/sgx: Expose SGX architectural definitions to the
>    kernel) to add MAINTAINER file update to include new introduced asm/sgx.h.
>  - Updated patch 13 (x86/sgx: Add helpers to expose ECREATE and EINIT to KVM)
>    to use addr and size directly in access_ok()s (which won't be triggered
>    anyway).
> 
> v1->v2:
> 
>  - No big change in design, structural of patch series, etc.
>  - Addressed Boris's comments regarding to suppressing both SGX1 and SGX2 in
>    /proc/cpuinfo, and improvement in feat_ctl.c when enabling SGX (patch 2
>    and 6).
>  - Addressed Sean's comments for both x86 part patches and KVM patches (patch 3,
>    5, 9, 12, 19, 21).
>  - Addressed Dave's comments in RFC v6 series (patch 13).
> 
> RFC->v1:
> 
>  - Refined patch (x86/sgx: Wipe out EREMOVE from sgx_free_epc_page()) to print
>    error msg that EPC page is leaked when EREMOVE failed, requested by Dave.
>  - Changelog history of all RFC series is removed in both this cover letter
>    and each individual patch, since majority of x86 part patches already got
>    Acked-by from Dave and Jarkko. And the changelogs are not quite useful from
>    my perspective.
> 
> =========
> KVM SGX virtualization Overview
> 
> - Virtual EPC
> 
> SGX enclave memory is special and is reserved specifically for enclave use.
> In bare-metal SGX enclaves, the kernel allocates enclave pages, copies data
> into the pages with privileged instructions, then allows the enclave to start.
> In this scenario, only initialized pages already assigned to an enclave are
> mapped to userspace.
> 
> In virtualized environments, the hypervisor still needs to do the physical
> enclave page allocation.  The guest kernel is responsible for the data copying
> (among other things).  This means that the job of starting an enclave is now
> split between hypervisor and guest.
> 
> This series introduces a new misc device: /dev/sgx_vepc.  This device allows
> the host to map *uninitialized* enclave memory into userspace, which can then
> be passed into a guest.
> 
> While it might be *possible* to start a host-side enclave with /dev/sgx_enclave
> and pass its memory into a guest, it would be wasteful and convoluted.
> 
> Implement the *raw* EPC allocation in the x86 core-SGX subsystem via
> /dev/sgx_vepc rather than in KVM.  Doing so has two major advantages:
> 
>   - Does not require changes to KVM's uAPI, e.g. EPC gets handled as
>     just another memory backend for guests.
> 
>   - EPC management is wholly contained in the SGX subsystem, e.g. SGX
>     does not have to export any symbols, changes to reclaim flows don't
>     need to be routed through KVM, SGX's dirty laundry doesn't have to
>     get aired out for the world to see, and so on and so forth.
> 
> The virtual EPC pages allocated to guests are currently not reclaimable.
> Reclaiming EPC page used by enclave requires a special reclaim mechanism
> separate from normal page reclaim, and that mechanism is not supported
> for virutal EPC pages.  Due to the complications of handling reclaim
> conflicts between guest and host, reclaiming virtual EPC pages is 
> significantly more complex than basic support for SGX virtualization.
> 
> - Support SGX virtualization without SGX Flexible Launch Control
> 
> SGX hardware supports two "launch control" modes to limit which enclaves can
> run.  In the "locked" mode, the hardware prevents enclaves from running unless
> they are blessed by a third party.  In the unlocked mode, the kernel is in
> full control of which enclaves can run.  The bare-metal SGX code refuses to
> launch enclaves unless it is in the unlocked mode.
> 
> This sgx_virt_epc driver does not have such a restriction.  This allows guests
> which are OK with the locked mode to use SGX, even if the host kernel refuses
> to.
> 
> - Support exposing SGX2
> 
> Due to the same reason above, SGX2 feature detection is added to core SGX code
> to allow KVM to expose SGX2 to guest, even currently SGX driver doesn't support
> SGX2, because SGX2 can work just fine in guest w/o any interaction to host SGX
> driver.
> 
> - Restricit SGX guest access to provisioning key
> 
> To grant guest being able to fully use SGX, guest needs to be able to access
> provisioning key.  The provisioning key is sensitive, and accessing to it should
> be restricted. In bare-metal driver, allowing enclave to access provisioning key
> is restricted by being able to open /dev/sgx_provision.
> 
> Add a new KVM_CAP_SGX_ATTRIBUTE to KVM uAPI to extend above mechanism to KVM
> guests as well.  When userspace hypervisor creates a new VM, the new cap is only
> added to VM when userspace hypervisior is able to open /dev/sgx_provision,
> following the same role as in bare-metal driver.  KVM then traps ECREATE from
> guest, and only allows ECREATE with provisioning key bit to run when guest
> supports KVM_CAP_SGX_ATTRIBUTE.
> 
> 
> Kai Huang (4):
>   x86/cpufeatures: Make SGX_LC feature bit depend on SGX bit
>   x86/sgx: Wipe out EREMOVE from sgx_free_epc_page()
>   x86/sgx: Initialize virtual EPC driver even when SGX driver is
>     disabled
>   x86/sgx: Add helper to update SGX_LEPUBKEYHASHn MSRs
> 
> Sean Christopherson (21):
>   x86/cpufeatures: Add SGX1 and SGX2 sub-features
>   x86/sgx: Add SGX_CHILD_PRESENT hardware error code
>   x86/sgx: Introduce virtual EPC for use by KVM guests
>   x86/cpu/intel: Allow SGX virtualization without Launch Control support
>   x86/sgx: Expose SGX architectural definitions to the kernel
>   x86/sgx: Move ENCLS leaf definitions to sgx.h
>   x86/sgx: Add SGX2 ENCLS leaf definitions (EAUG, EMODPR and EMODT)
>   x86/sgx: Add encls_faulted() helper
>   x86/sgx: Add helpers to expose ECREATE and EINIT to KVM
>   x86/sgx: Move provisioning device creation out of SGX driver
>   KVM: x86: Export kvm_mmu_gva_to_gpa_{read,write}() for SGX (VMX)
>   KVM: x86: Define new #PF SGX error code bit
>   KVM: x86: Add support for reverse CPUID lookup of scattered features
>   KVM: x86: Add reverse-CPUID lookup support for scattered SGX features
>   KVM: VMX: Add basic handling of VM-Exit from SGX enclave
>   KVM: VMX: Frame in ENCLS handler for SGX virtualization
>   KVM: VMX: Add SGX ENCLS[ECREATE] handler to enforce CPUID restrictions
>   KVM: VMX: Add emulation of SGX Launch Control LE hash MSRs
>   KVM: VMX: Add ENCLS[EINIT] handler to support SGX Launch Control (LC)
>   KVM: VMX: Enable SGX virtualization for SGX1, SGX2 and LC
>   KVM: x86: Add capability to grant VM access to privileged SGX
>     attribute
> 
>  Documentation/virt/kvm/api.rst                |  23 +
>  MAINTAINERS                                   |   1 +
>  arch/x86/Kconfig                              |  12 +
>  arch/x86/include/asm/cpufeatures.h            |   2 +
>  arch/x86/include/asm/kvm_host.h               |   5 +
>  .../cpu/sgx/arch.h => include/asm/sgx.h}      |  50 +-
>  arch/x86/include/asm/vmx.h                    |   1 +
>  arch/x86/include/uapi/asm/vmx.h               |   1 +
>  arch/x86/kernel/cpu/cpuid-deps.c              |   3 +
>  arch/x86/kernel/cpu/feat_ctl.c                |  71 ++-
>  arch/x86/kernel/cpu/scattered.c               |   2 +
>  arch/x86/kernel/cpu/sgx/Makefile              |   1 +
>  arch/x86/kernel/cpu/sgx/driver.c              |  17 -
>  arch/x86/kernel/cpu/sgx/encl.c                |  42 +-
>  arch/x86/kernel/cpu/sgx/encl.h                |   1 +
>  arch/x86/kernel/cpu/sgx/encls.h               |  30 +-
>  arch/x86/kernel/cpu/sgx/ioctl.c               |  29 +-
>  arch/x86/kernel/cpu/sgx/main.c                |  96 +++-
>  arch/x86/kernel/cpu/sgx/sgx.h                 |  13 +-
>  arch/x86/kernel/cpu/sgx/virt.c                | 369 ++++++++++++++
>  arch/x86/kvm/Makefile                         |   2 +
>  arch/x86/kvm/cpuid.c                          |  89 +++-
>  arch/x86/kvm/cpuid.h                          |  50 +-
>  arch/x86/kvm/vmx/nested.c                     |  28 +-
>  arch/x86/kvm/vmx/nested.h                     |   5 +
>  arch/x86/kvm/vmx/sgx.c                        | 481 ++++++++++++++++++
>  arch/x86/kvm/vmx/sgx.h                        |  34 ++
>  arch/x86/kvm/vmx/vmcs12.c                     |   1 +
>  arch/x86/kvm/vmx/vmcs12.h                     |   4 +-
>  arch/x86/kvm/vmx/vmx.c                        | 109 +++-
>  arch/x86/kvm/vmx/vmx.h                        |   2 +
>  arch/x86/kvm/x86.c                            |  23 +
>  include/uapi/linux/kvm.h                      |   1 +
>  tools/testing/selftests/sgx/defines.h         |   2 +-
>  34 files changed, 1476 insertions(+), 124 deletions(-)
>  rename arch/x86/{kernel/cpu/sgx/arch.h => include/asm/sgx.h} (89%)
>  create mode 100644 arch/x86/kernel/cpu/sgx/virt.c
>  create mode 100644 arch/x86/kvm/vmx/sgx.c
>  create mode 100644 arch/x86/kvm/vmx/sgx.h
> 
> -- 
> 2.30.2
> 
> 

I just say add my ack to SGX specific patches where it is missing.
Good enough.

/Jarkko

  parent reply	other threads:[~2021-03-19 14:53 UTC|newest]

Thread overview: 124+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-19  7:29 [PATCH v3 00/25] KVM SGX virtualization support Kai Huang
2021-03-19  7:22 ` [PATCH v3 01/25] x86/cpufeatures: Make SGX_LC feature bit depend on SGX bit Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Kai Huang
2021-03-19  7:22 ` [PATCH v3 02/25] x86/cpufeatures: Add SGX1 and SGX2 sub-features Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:22 ` [PATCH v3 03/25] x86/sgx: Wipe out EREMOVE from sgx_free_epc_page() Kai Huang
2021-03-22 18:16   ` Borislav Petkov
2021-03-22 18:56     ` Sean Christopherson
2021-03-22 19:11       ` Paolo Bonzini
2021-03-22 20:43         ` Kai Huang
2021-03-23 16:40           ` Paolo Bonzini
2021-03-22 19:15       ` Borislav Petkov
2021-03-22 19:37         ` Sean Christopherson
2021-03-22 20:36           ` Kai Huang
2021-03-22 21:06           ` Borislav Petkov
2021-03-22 22:06             ` Kai Huang
2021-03-22 22:37               ` Borislav Petkov
2021-03-22 23:16                 ` Kai Huang
2021-03-23 15:45                   ` Sean Christopherson
2021-03-23 16:06                     ` Borislav Petkov
2021-03-23 16:21                       ` Sean Christopherson
2021-03-23 16:32                         ` Borislav Petkov
2021-03-23 16:51                           ` Sean Christopherson
2021-03-24  9:38                           ` Kai Huang
2021-03-24 10:09                             ` Paolo Bonzini
2021-03-24 10:48                               ` Kai Huang
2021-03-24 11:24                                 ` Paolo Bonzini
2021-03-24 23:23                               ` Kai Huang
2021-03-24 23:39                                 ` Paolo Bonzini
2021-03-24 23:46                                   ` Kai Huang
2021-03-25  8:42                                     ` Borislav Petkov
2021-03-25  9:38                                       ` Kai Huang
2021-03-25 16:52                                         ` Borislav Petkov
2021-03-24  9:28                         ` Jarkko Sakkinen
2021-03-23 16:38                       ` Paolo Bonzini
2021-03-23 17:02                         ` Sean Christopherson
2021-03-23 17:06                           ` Paolo Bonzini
2021-03-23 17:16                             ` Sean Christopherson
2021-03-23 18:16                             ` Borislav Petkov
2021-03-24  9:26                       ` Jarkko Sakkinen
2021-03-22 22:23             ` Kai Huang
2021-03-25  9:30   ` [PATCH v4 " Kai Huang
2021-03-26 19:48     ` Jarkko Sakkinen
2021-03-26 20:38       ` Kai Huang
2021-03-26 21:39       ` Jarkko Sakkinen
2021-04-07 10:03     ` [tip: x86/sgx] " tip-bot2 for Kai Huang
2021-03-19  7:22 ` [PATCH v3 04/25] x86/sgx: Add SGX_CHILD_PRESENT hardware error code Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:22 ` [PATCH v3 05/25] x86/sgx: Introduce virtual EPC for use by KVM guests Kai Huang
2021-03-25  9:36   ` Kai Huang
2021-03-26 15:03   ` Borislav Petkov
2021-03-26 15:17     ` Dave Hansen
2021-03-26 15:29       ` Borislav Petkov
2021-03-26 15:35         ` Dave Hansen
2021-03-26 17:02           ` Borislav Petkov
2021-03-31  1:10     ` Kai Huang
2021-03-31  6:44       ` Boris Petkov
2021-03-31  6:51         ` Kai Huang
2021-03-31  7:44           ` Boris Petkov
2021-03-31  8:53             ` Kai Huang
2021-03-31 12:20               ` Kai Huang
2021-04-01 18:31                 ` Borislav Petkov
2021-04-01 23:38                   ` Kai Huang
2021-04-01  9:45               ` Kai Huang
2021-04-01  9:42   ` [PATCH v4 " Kai Huang
2021-04-05  9:01   ` [PATCH v3 " Borislav Petkov
2021-04-05 21:46     ` Kai Huang
2021-04-06  8:28       ` Borislav Petkov
2021-04-06  9:04         ` Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:22 ` [PATCH v3 06/25] x86/cpu/intel: Allow SGX virtualization without Launch Control support Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:23 ` [PATCH v3 07/25] x86/sgx: Initialize virtual EPC driver even when SGX driver is disabled Kai Huang
2021-04-02  9:48   ` Borislav Petkov
2021-04-02 11:08     ` Kai Huang
2021-04-02 11:22       ` Borislav Petkov
2021-04-02 11:38         ` Kai Huang
2021-04-02 15:42     ` Sean Christopherson
2021-04-02 19:08       ` Kai Huang
2021-04-02 19:19       ` Borislav Petkov
2021-04-02 19:30         ` Sean Christopherson
2021-04-02 19:46           ` Borislav Petkov
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Kai Huang
2021-03-19  7:23 ` [PATCH v3 08/25] x86/sgx: Expose SGX architectural definitions to the kernel Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:23 ` [PATCH v3 09/25] x86/sgx: Move ENCLS leaf definitions to sgx.h Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:23 ` [PATCH v3 10/25] x86/sgx: Add SGX2 ENCLS leaf definitions (EAUG, EMODPR and EMODT) Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:23 ` [PATCH v3 11/25] x86/sgx: Add encls_faulted() helper Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:23 ` [PATCH v3 12/25] x86/sgx: Add helper to update SGX_LEPUBKEYHASHn MSRs Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Kai Huang
2021-03-19  7:23 ` [PATCH v3 13/25] x86/sgx: Add helpers to expose ECREATE and EINIT to KVM Kai Huang
2021-04-05  9:07   ` Borislav Petkov
2021-04-05 21:44     ` Kai Huang
2021-04-06  7:40       ` Borislav Petkov
2021-04-06  8:59         ` Kai Huang
2021-04-06  9:09           ` Borislav Petkov
2021-04-06  9:24             ` Kai Huang
2021-04-06  9:32               ` Borislav Petkov
2021-04-06  9:41                 ` Kai Huang
2021-04-06 17:08                   ` Borislav Petkov
2021-04-06 20:33                     ` Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:23 ` [PATCH v3 14/25] x86/sgx: Move provisioning device creation out of SGX driver Kai Huang
2021-04-07 10:03   ` [tip: x86/sgx] " tip-bot2 for Sean Christopherson
2021-03-19  7:23 ` [PATCH v3 15/25] KVM: x86: Export kvm_mmu_gva_to_gpa_{read,write}() for SGX (VMX) Kai Huang
2021-03-19  7:23 ` [PATCH v3 16/25] KVM: x86: Define new #PF SGX error code bit Kai Huang
2021-03-19  7:23 ` [PATCH v3 17/25] KVM: x86: Add support for reverse CPUID lookup of scattered features Kai Huang
2021-03-19  7:23 ` [PATCH v3 18/25] KVM: x86: Add reverse-CPUID lookup support for scattered SGX features Kai Huang
2021-03-19  7:23 ` [PATCH v3 19/25] KVM: VMX: Add basic handling of VM-Exit from SGX enclave Kai Huang
2021-03-19  7:23 ` [PATCH v3 20/25] KVM: VMX: Frame in ENCLS handler for SGX virtualization Kai Huang
2021-03-19  7:23 ` [PATCH v3 21/25] KVM: VMX: Add SGX ENCLS[ECREATE] handler to enforce CPUID restrictions Kai Huang
2021-03-19  7:23 ` [PATCH v3 22/25] KVM: VMX: Add emulation of SGX Launch Control LE hash MSRs Kai Huang
2021-03-19  7:23 ` [PATCH v3 23/25] KVM: VMX: Add ENCLS[EINIT] handler to support SGX Launch Control (LC) Kai Huang
2021-03-19  7:23 ` [PATCH v3 24/25] KVM: VMX: Enable SGX virtualization for SGX1, SGX2 and LC Kai Huang
2021-03-19  7:24 ` [PATCH v3 25/25] KVM: x86: Add capability to grant VM access to privileged SGX attribute Kai Huang
2021-03-19 14:52 ` Jarkko Sakkinen [this message]
2021-03-22 10:03   ` [PATCH v3 00/25] KVM SGX virtualization support Kai Huang
2021-03-22 10:31     ` Borislav Petkov
2021-03-26 22:46 ` Jarkko Sakkinen
2021-03-28 21:01   ` Huang, Kai
2021-03-31 23:23     ` Jarkko Sakkinen

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=YFS6kTe1SuAjiMFN@kernel.org \
    --to=jarkko@kernel.org \
    --cc=b.thiel@posteo.de \
    --cc=bp@alien8.de \
    --cc=corbet@lwn.net \
    --cc=dave.hansen@intel.com \
    --cc=haitao.huang@intel.com \
    --cc=hpa@zytor.com \
    --cc=jethro@fortanix.com \
    --cc=jmattson@google.com \
    --cc=joro@8bytes.org \
    --cc=kai.huang@intel.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-sgx@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=mingo@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=rick.p.edgecombe@intel.com \
    --cc=seanjc@google.com \
    --cc=tglx@linutronix.de \
    --cc=vkuznets@redhat.com \
    --cc=wanpengli@tencent.com \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).