linux-kernel - Re: [V1 PATCH 1/6] KVM: x86: Add support for testing private memory

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Date:   Wed, 23 Nov 2022 17:49:38 -0800
From:   Marc Orr <marcorr@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>,
        Vishal Annapurve <vannapurve@...gle.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, pbonzini@...hat.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
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        aaronlewis@...gle.com, wei.w.wang@...el.com,
        kirill.shutemov@...ux.intel.com, corbet@....net, hughd@...gle.com,
        jlayton@...nel.org, bfields@...ldses.org,
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Subject: Re: [V1 PATCH 1/6] KVM: x86: Add support for testing private memory

On Tue, Nov 22, 2022 at 12:06 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Nov 22, 2022, Chao Peng wrote:
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 10017a9f26ee..b3118d00b284 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -4280,6 +4280,10 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> > >
> > >     fault->gfn = fault->addr >> PAGE_SHIFT;
> > >     fault->slot = kvm_vcpu_gfn_to_memslot(vcpu, fault->gfn);
> > > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING
> > > +   fault->is_private = kvm_slot_can_be_private(fault->slot) &&
> > > +                   kvm_mem_is_private(vcpu->kvm, fault->gfn);
> > > +#endif
> > >
> > >     if (page_fault_handle_page_track(vcpu, fault))
> > >             return RET_PF_EMULATE;
> > > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > > index 5cdff5ca546c..2e759f39c2c5 100644
> > > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > > @@ -188,7 +188,6 @@ struct kvm_page_fault {
> > >
> > >     /* Derived from mmu and global state.  */
> > >     const bool is_tdp;
> > > -   const bool is_private;
> > >     const bool nx_huge_page_workaround_enabled;
> > >
> > >     /*
> > > @@ -221,6 +220,9 @@ struct kvm_page_fault {
> > >     /* The memslot containing gfn. May be NULL. */
> > >     struct kvm_memory_slot *slot;
> > >
> > > +   /* Derived from encryption bits of the faulting GPA for CVMs. */
> > > +   bool is_private;
> >
> > Either we can wrap it with the CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING or if
> > it looks ugly I can remove the "const" in my code.
>
> Hmm, I think we can keep the const.  Similar to the bug in kvm_faultin_pfn()[*],
> the kvm_slot_can_be_private() is bogus.  A fault should be considered private if
> it's marked as private, whether or not userspace has configured the slot to be
> private is irrelevant.  I.e. the xarray is the single source of truth, memslots
> are just plumbing.

If we incorporate Sean's suggestion and use xarray as the single
source of truth, then can we get rid of the
HAVE_KVM_PRIVATE_MEM_TESTING config?

Specifically, the self test can call the KVM_MEMORY_ENCRYPT_REG_REGION
ioctl which will set the bits for the private FD within KVM's xarray.

(Maybe this was part of the point that Sean was making; but his
feedback seemed focused on the discussion about keeping `is_private`
const, whereas I've been staring at this trying to figure out if we
can run the UPM selftests on a non-TDX/SNP VM WITHOUT a special
test-only config. And Sean's idea seems to eliminate the need for the
awkward CONFIG.)