Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM

Isaku Yamahata
2023-01-19T22:55:15Z

On Thu, Jan 19, 2023 at 03:25:08PM +0000,
Sean Christopherson <seanjc@google.com> wrote:

> On Thu, Jan 19, 2023, Isaku Yamahata wrote:
> > On Sat, Jan 14, 2023 at 12:37:59AM +0000,
> > Sean Christopherson <seanjc@google.com> wrote:
> >
> > > On Fri, Dec 02, 2022, Chao Peng wrote:
> > > > This patch series implements KVM guest private memory for confidential
> > > > computing scenarios like Intel TDX[1]. If a TDX host accesses
> > > > TDX-protected guest memory, machine check can happen which can further
> > > > crash the running host system, this is terrible for multi-tenant
> > > > configurations. The host accesses include those from KVM userspace like
> > > > QEMU. This series addresses KVM userspace induced crash by introducing
> > > > new mm and KVM interfaces so KVM userspace can still manage guest memory
> > > > via a fd-based approach, but it can never access the guest memory
> > > > content.
> > > >
> > > > The patch series touches both core mm and KVM code. I appreciate
> > > > Andrew/Hugh and Paolo/Sean can review and pick these patches. Any other
> > > > reviews are always welcome.
> > > > - 01: mm change, target for mm tree
> > > > - 02-09: KVM change, target for KVM tree
> > >
> > > A version with all of my feedback, plus reworked versions of Vishal's selftest,
> > > is available here:
> > >
> > > git@github.com:sean-jc/linux.git x86/upm_base_support
> > >
> > > It compiles and passes the selftest, but it's otherwise barely tested. There are
> > > a few todos (2 I think?) and many of the commits need changelogs, i.e. it's still
> > > a WIP.
> > >
> > > As for next steps, can you (handwaving all of the TDX folks) take a look at what
> > > I pushed and see if there's anything horrifically broken, and that it still works
> > > for TDX?
> > >
> > > Fuad (and pKVM folks) same ask for you with respect to pKVM. Absolutely no rush
> > > (and I mean that).
> > >
> > > On my side, the two things on my mind are (a) tests and (b) downstream dependencies
> > > (SEV and TDX). For tests, I want to build a lists of tests that are required for
> > > merging so that the criteria for merging are clear, and so that if the list is large
> > > (haven't thought much yet), the work of writing and running tests can be distributed.
> > >
> > > Regarding downstream dependencies, before this lands, I want to pull in all the
> > > TDX and SNP series and see how everything fits together. Specifically, I want to
> > > make sure that we don't end up with a uAPI that necessitates ugly code, and that we
> > > don't miss an opportunity to make things simpler. The patches in the SNP series to
> > > add "legacy" SEV support for UPM in particular made me slightly rethink some minor
> > > details. Nothing remotely major, but something that needs attention since it'll
> > > be uAPI.
> >
> > Although I'm still debuging with TDX KVM, I needed the following.
> > kvm_faultin_pfn() is called without mmu_lock held. the race to change
> > private/shared is handled by mmu_seq. Maybe dedicated function only for
> > kvm_faultin_pfn().
>
> Gah, you're not on the other thread where this was discussed[*]. Simply deleting
> the lockdep assertion is safe, for guest types that rely on the attributes to
> define shared vs. private, KVM rechecks the attributes under the protection of
> mmu_seq.
>
> I'll get a fixed version pushed out today.
>
> [*] https://lore.kernel.org/all/Y8gpl+LwSuSgBFks@google.com

Now I have tdx kvm working. I've uploaded at the followings.
It's rebased to v6.2-rc3.
git@github.com:yamahata/linux.git tdx/upm
git@github.com:yamahata/qemu.git tdx/upm

kvm_mmu_do_page_fault() needs the following change.
kvm_mem_is_private() queries mem_attr_array. kvm_faultin_pfn() also uses
kvm_mem_is_private(). So the shared-private check in kvm_faultin_pfn() doesn't
make sense. This change would belong to TDX KVM patches, though.

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 72b0da8e27e0..f45ac438bbf4 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -430,7 +430,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
.max_level = vcpu->kvm->arch.tdp_max_page_level,
.req_level = PG_LEVEL_4K,
.goal_level = PG_LEVEL_4K,
- .is_private = kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT),
+ .is_private = kvm_is_private_gpa(vcpu->kvm, cr2_or_gpa),
};
int r;

--
Isaku Yamahata <isaku.yamahata@gmail.com>