Re: [PATCH Part2 v6 41/49] KVM: SVM: Add support to handle the RMP nested page fault

Alper Gun
2022-10-12T22:53:56Z

On Mon, Oct 10, 2022 at 7:32 PM Kalra, Ashish <ashish.kalra@amd.com> wrote:
>
> Hello Alper,
>
> On 10/10/2022 5:03 PM, Alper Gun wrote:
> > On Mon, Jun 20, 2022 at 4:13 PM Ashish Kalra <Ashish.Kalra@amd.com> wrote:
> >>
> >> From: Brijesh Singh <brijesh.singh@amd.com>
> >>
> >> When SEV-SNP is enabled in the guest, the hardware places restrictions on
> >> all memory accesses based on the contents of the RMP table. When hardware
> >> encounters RMP check failure caused by the guest memory access it raises
> >> the #NPF. The error code contains additional information on the access
> >> type. See the APM volume 2 for additional information.
> >>
> >> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> >> ---
> >> arch/x86/kvm/svm/sev.c | 76 ++++++++++++++++++++++++++++++++++++++++++
> >> arch/x86/kvm/svm/svm.c | 14 +++++---
> >> 2 files changed, 86 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> >> index 4ed90331bca0..7fc0fad87054 100644
> >> --- a/arch/x86/kvm/svm/sev.c
> >> +++ b/arch/x86/kvm/svm/sev.c
> >> @@ -4009,3 +4009,79 @@ void sev_post_unmap_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn)
> >>
> >> spin_unlock(&sev->psc_lock);
> >> }
> >> +
> >> +void handle_rmp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
> >> +{
> >> + int rmp_level, npt_level, rc, assigned;
> >> + struct kvm *kvm = vcpu->kvm;
> >> + gfn_t gfn = gpa_to_gfn(gpa);
> >> + bool need_psc = false;
> >> + enum psc_op psc_op;
> >> + kvm_pfn_t pfn;
> >> + bool private;
> >> +
> >> + write_lock(&kvm->mmu_lock);
> >> +
> >> + if (unlikely(!kvm_mmu_get_tdp_walk(vcpu, gpa, &pfn, &npt_level)))
> >> + goto unlock;
> >> +
> >> + assigned = snp_lookup_rmpentry(pfn, &rmp_level);
> >> + if (unlikely(assigned < 0))
> >> + goto unlock;
> >> +
> >> + private = !!(error_code & PFERR_GUEST_ENC_MASK);
> >> +
> >> + /*
> >> + * If the fault was due to size mismatch, or NPT and RMP page level's
> >> + * are not in sync, then use PSMASH to split the RMP entry into 4K.
> >> + */
> >> + if ((error_code & PFERR_GUEST_SIZEM_MASK) ||
> >> + (npt_level == PG_LEVEL_4K && rmp_level == PG_LEVEL_2M && private)) {
> >> + rc = snp_rmptable_psmash(kvm, pfn);
> >
> >
> > Regarding this case:
> > RMP level is 4K
> > Page table level is 2M
> >
> > Does this also cause a page fault with size mismatch? If so, we
> > shouldn't try psmash because the rmp entry is already 4K.
> >
> > I see these errors in our tests and I think it may be happening
> > because rmp size is already 4K.
> >
> > [ 1848.752952] psmash failed, gpa 0x191560000 pfn 0x536cd60 rc 7
> > [ 2922.879635] psmash failed, gpa 0x102830000 pfn 0x37c8230 rc 7
> > [ 3010.983090] psmash failed, gpa 0x104220000 pfn 0x6cf1e20 rc 7
> > [ 3170.792050] psmash failed, gpa 0x108a80000 pfn 0x20e0080 rc 7
> > [ 3345.955147] psmash failed, gpa 0x11b480000 pfn 0x1545e480 rc 7
> >
> > Shouldn't we use AND instead of OR in the if statement?
> >
>
> I believe this we can't do, looking at the typical usage case below :
>
> [ 37.243969] #VMEXIT (NPF) - SIZEM, err 0xc80000005 npt_level 2,
> rmp_level 2, private 1
> [ 37.243973] trying psmash gpa 0x7f790000 pfn 0x1f5d90
>
> This is typically the case with #VMEXIT(NPF) with SIZEM error code, when
> the guest tries to do PVALIDATE on 4K GHCB pages, in this case both the
> RMP table and NPT will be optimally setup to 2M hugepage as can be seen.
>
> Is it possible to investigate in more depth, when is the this case being
> observed:

Yes, I added more logs and I can see that these errors happen when RMP
level is 4K and NPT level is 2M.
psmash fails as expected. I think it is just a log, there is no real
issue but the best is not trying psmash if rmp level is 4K.

> RMP level is 4K
> Page table level is 2M
> We shouldn't try psmash because the rmp entry is already 4K.
>
> Thanks,
> Ashish
>
> > if ((error_code & PFERR_GUEST_SIZEM_MASK) && ...
> >