From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Isaku Yamahata <isaku.yamahata@intel.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: Thu, 25 Apr 2024 09:00:38 -0700 [thread overview]
Message-ID: <Zip-JsAB5TIRDJVl@google.com> (raw)
In-Reply-To: <CABgObfY2TOb6cJnFkpxWjkAmbYSRGkXGx=+-241tRx=OG-yAZQ@mail.gmail.com>
On Thu, Apr 25, 2024, Paolo Bonzini wrote:
> On Thu, Apr 25, 2024 at 3:12 AM Isaku Yamahata <isaku.yamahata@intel.com> wrote:
> > > > 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.
>
> The invalidate_lock is only needed to operate on the guest_memfd, but
> it's a rwsem so there are no risks of lock inversion.
>
> > > 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.
>
> I don't understand why TDH.MEM.PAGE.ADD() cannot be called from the
> post-populate hook. Can the code for TDH.MEM.PAGE.ADD be shared
> between the memory initialization ioctl and the page fault hook in
> kvm_x86_ops?
Ah, because TDX is required to pre-fault the memory to establish the S-EPT walk,
and pre-faulting means guest_memfd()
Requiring that guest_memfd not have a page when initializing the guest image
seems wrong, i.e. I don't think we want FGP_CREAT_ONLY. And not just because I
am a fan of pre-faulting, I think the semantics are bad.
E.g. IIUC, doing fallocate() to ensure memory is available would cause LAUNCH_UPDATE
to fail. That's weird and has nothing to do with KVM_PRE_FAULT.
I don't understand why we want FGP_CREAT_ONLY semantics. Who cares if there's a
page allocated? KVM already checks that the page is unassigned in the RMP, so
why does guest_memfd care whether or not the page was _just_ allocated?
AFAIK, unwinding on failure is completely uninteresting, and arguably undesirable,
because undoing LAUNCH_UPDATE or PAGE.ADD will affect the measurement, i.e. there
is no scenario where deleting pages from guest_memfd would allow a restart/resume
of the build process to truly succeed.
next prev parent reply other threads:[~2024-04-25 16:00 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
2024-04-25 6:01 ` Paolo Bonzini
2024-04-25 16:00 ` Sean Christopherson [this message]
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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