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

[Sean Christopherson <seanjc@google.com>]

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.

Then kvm_mmu_do_page_fault() can do something like:

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index dbaf6755c5a7..456a9daa36e5 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -260,6 +260,8 @@ enum {
 static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
                                        u32 err, bool prefetch)
 {
+       bool is_tdp = likely(vcpu->arch.mmu->page_fault == kvm_tdp_page_fault);
+
        struct kvm_page_fault fault = {
                .addr = cr2_or_gpa,
                .error_code = err,
@@ -269,13 +271,15 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
                .rsvd = err & PFERR_RSVD_MASK,
                .user = err & PFERR_USER_MASK,
                .prefetch = prefetch,
-               .is_tdp = likely(vcpu->arch.mmu->page_fault == kvm_tdp_page_fault),
+               .is_tdp = is_tdp,
                .nx_huge_page_workaround_enabled =
                        is_nx_huge_page_enabled(vcpu->kvm),
 
                .max_level = KVM_MAX_HUGEPAGE_LEVEL,
                .req_level = PG_LEVEL_4K,
                .goal_level = PG_LEVEL_4K,
+               .private = IS_ENABLED(CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING) && is_tdp &&
+                          kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT),
        };
        int r;

[*] https://lore.kernel.org/all/Y3Vgc5KrNRA8r6vh@google.com