Re: [PATCH v4 1/9] KVM: x86/mmu: Bug the VM if KVM attempts to double count an NX huge page

Sean Christopherson
2022-09-21T15:42:09Z

On Wed, Sep 21, 2022, Sean Christopherson wrote:
> On Wed, Sep 21, 2022, Vitaly Kuznetsov wrote:
> > [ 962.257992] ept_fetch+0x504/0x5a0 [kvm]
> > [ 962.261959] ept_page_fault+0x2d7/0x300 [kvm]
> > [ 962.287701] kvm_mmu_page_fault+0x258/0x290 [kvm]
> > [ 962.292451] vmx_handle_exit+0xe/0x40 [kvm_intel]
> > [ 962.297173] vcpu_enter_guest+0x665/0xfc0 [kvm]
> > [ 962.307580] vcpu_run+0x33/0x250 [kvm]
> > [ 962.311367] kvm_arch_vcpu_ioctl_run+0xf7/0x460 [kvm]
> > [ 962.316456] kvm_vcpu_ioctl+0x271/0x670 [kvm]
> > [ 962.320843] __x64_sys_ioctl+0x87/0xc0
> > [ 962.324602] do_syscall_64+0x38/0x90
> > [ 962.328192] entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> Ugh, past me completely forgot the basics of shadow paging[*]. The shadow MMU
> can reuse existing shadow pages, whereas the TDP MMU always links in new pages.
>
> I got turned around by the "doesn't exist" check, which only means "is there
> already a _SPTE_ here", not "is there an existing SP for the target gfn+role that
> can be used".
>
> I'll drop the series from the queue, send a new pull request, and spin a v5
> targeting 6.2, which amusing will look a lot like v1...

Huh. I was expecting more churn, but dropping the offending patch and then
"reworking" the series yields a very trivial overall diff.

Vitaly, can you easily re-test with the below, i.e. simply delete the KVM_BUG_ON()?
I'll still spin a v5, but assuming all is well I think this can go into 6.1 and
not get pushed out to 6.2.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 54ee48a87f81..e6f19e605979 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -804,7 +804,15 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)

void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
{
- if (KVM_BUG_ON(!list_empty(&sp->possible_nx_huge_page_link), kvm))
+ /*
+ * If it's possible to replace the shadow page with an NX huge page,
+ * i.e. if the shadow page is the only thing currently preventing KVM
+ * from using a huge page, add the shadow page to the list of "to be
+ * zapped for NX recovery" pages. Note, the shadow page can already be
+ * on the list if KVM is reusing an existing shadow page, i.e. if KVM
+ * links a shadow page at multiple points.
+ */
+ if (!list_empty(&sp->possible_nx_huge_page_link))
return;

++kvm->stat.nx_lpage_splits;