From: Isaku Yamahata <isaku.yamahata@intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Isaku Yamahata <isaku.yamahata@intel.com>,
Paolo Bonzini <pbonzini@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
michael.roth@amd.com, isaku.yamahata@linux.intel.com
Subject: Re: [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem pages with user data
Date: Wed, 24 Apr 2024 18:12:48 -0700 [thread overview]
Message-ID: <20240425011248.GP3596705@ls.amr.corp.intel.com> (raw)
In-Reply-To: <ZimGulY6qyxt6ylO@google.com>
On Wed, Apr 24, 2024 at 03:24:58PM -0700,
Sean Christopherson <seanjc@google.com> wrote:
> On Tue, Apr 23, 2024, Isaku Yamahata wrote:
> > On Thu, Apr 04, 2024 at 02:50:31PM -0400,
> > Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > > During guest run-time, kvm_arch_gmem_prepare() is issued as needed to
> > > prepare newly-allocated gmem pages prior to mapping them into the guest.
> > > In the case of SEV-SNP, this mainly involves setting the pages to
> > > private in the RMP table.
> > >
> > > However, for the GPA ranges comprising the initial guest payload, which
> > > are encrypted/measured prior to starting the guest, the gmem pages need
> > > to be accessed prior to setting them to private in the RMP table so they
> > > can be initialized with the userspace-provided data. Additionally, an
> > > SNP firmware call is needed afterward to encrypt them in-place and
> > > measure the contents into the guest's launch digest.
> > >
> > > While it is possible to bypass the kvm_arch_gmem_prepare() hooks so that
> > > this handling can be done in an open-coded/vendor-specific manner, this
> > > may expose more gmem-internal state/dependencies to external callers
> > > than necessary. Try to avoid this by implementing an interface that
> > > tries to handle as much of the common functionality inside gmem as
> > > possible, while also making it generic enough to potentially be
> > > usable/extensible for TDX as well.
> >
> > I explored how TDX will use this hook. However, it resulted in not using this
> > hook, and instead used kvm_tdp_mmu_get_walk() with a twist. The patch is below.
> >
> > Because SEV-SNP manages the RMP that is not tied to NPT directly, SEV-SNP can
> > ignore TDP MMU page tables when updating RMP.
> > On the other hand, TDX essentially updates Secure-EPT when it adds a page to
> > the guest by TDH.MEM.PAGE.ADD(). It needs to protect KVM TDP MMU page tables
> > with mmu_lock, not guest memfd file mapping with invalidate_lock. The hook
> > doesn't apply to TDX well. The resulted KVM_TDX_INIT_MEM_REGION logic is as
> > follows.
> >
> > get_user_pages_fast(source addr)
> > read_lock(mmu_lock)
> > kvm_tdp_mmu_get_walk_private_pfn(vcpu, gpa, &pfn);
> > if the page table doesn't map gpa, error.
> > TDH.MEM.PAGE.ADD()
> > TDH.MR.EXTEND()
> > read_unlock(mmu_lock)
> > put_page()
>
> Hmm, KVM doesn't _need_ to use invalidate_lock to protect against guest_memfd
> invalidation, but I also don't see why it would cause problems. I.e. why not
> take mmu_lock() in TDX's post_populate() implementation?
We can take the lock. Because we have already populated the GFN of guest_memfd,
we need to make kvm_gmem_populate() not pass FGP_CREAT_ONLY. Otherwise we'll
get -EEXIST.
> That would allow having
> a sanity check that the PFN that guest_memfd() has is indeed the PFN that KVM's
> S-EPT mirror has, i.e. the PFN that KVM is going to PAGE.ADD.
Because we have PFN from the mirrored EPT, I thought it's duplicate to get PFN
again via guest memfd. We can check if two PFN matches.
--
Isaku Yamahata <isaku.yamahata@intel.com>
next prev parent reply other threads:[~2024-04-25 1:12 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-04 18:50 [PATCH 00/11] KVM: guest_memfd: New hooks and functionality for SEV-SNP and TDX Paolo Bonzini
2024-04-04 18:50 ` [PATCH 01/11] mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory Paolo Bonzini
2024-04-29 13:14 ` Vlastimil Babka
2024-04-04 18:50 ` [PATCH 02/11] KVM: guest_memfd: Use AS_INACCESSIBLE when creating guest_memfd inode Paolo Bonzini
2024-04-29 13:15 ` Vlastimil Babka
2024-04-04 18:50 ` [PATCH 03/11] KVM: guest_memfd: pass error up from filemap_grab_folio Paolo Bonzini
2024-04-04 18:50 ` [PATCH 04/11] filemap: add FGP_CREAT_ONLY Paolo Bonzini
2024-04-25 5:52 ` Paolo Bonzini
2024-04-29 13:26 ` Vlastimil Babka
2024-04-04 18:50 ` [PATCH 05/11] KVM: guest_memfd: limit overzealous WARN Paolo Bonzini
2024-04-04 18:50 ` [PATCH 06/11] KVM: guest_memfd: Add hook for initializing memory Paolo Bonzini
2024-04-22 10:53 ` Xu Yilun
2024-05-07 16:17 ` Paolo Bonzini
2024-04-04 18:50 ` [PATCH 07/11] KVM: guest_memfd: extract __kvm_gmem_get_pfn() Paolo Bonzini
2024-04-09 23:35 ` Michael Roth
2024-04-24 22:34 ` Sean Christopherson
2024-04-24 22:59 ` Sean Christopherson
2024-04-04 18:50 ` [PATCH 08/11] KVM: guest_memfd: extract __kvm_gmem_punch_hole() Paolo Bonzini
2024-04-04 18:50 ` [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem pages with user data Paolo Bonzini
2024-04-22 14:44 ` Xu Yilun
2024-04-23 23:50 ` Isaku Yamahata
2024-04-24 22:24 ` Sean Christopherson
2024-04-25 1:12 ` Isaku Yamahata [this message]
2024-04-25 6:01 ` Paolo Bonzini
2024-04-25 16:00 ` Sean Christopherson
2024-04-25 16:51 ` Isaku Yamahata
2024-04-26 5:44 ` Paolo Bonzini
2024-04-26 17:15 ` Isaku Yamahata
2024-04-26 5:41 ` Paolo Bonzini
2024-04-26 15:17 ` Sean Christopherson
2024-04-24 22:32 ` Sean Christopherson
2024-04-25 5:56 ` Paolo Bonzini
2024-04-04 18:50 ` [PATCH 10/11] KVM: guest_memfd: Add hook for invalidating memory Paolo Bonzini
2024-04-04 18:50 ` [PATCH 11/11] KVM: x86: Add gmem hook for determining max NPT mapping level Paolo Bonzini
2024-04-09 23:46 ` Michael Roth
2024-04-19 18:26 ` Isaku Yamahata
2024-04-22 14:52 ` Xu Yilun
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