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

Vitaly Kuznetsov
2022-09-21T16:28:05Z

Sean Christopherson <seanjc@google.com> writes:

> 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()?

This seems to work! At least, I haven't noticed anything weird when
booting my beloved Win11 + WSL2 guest.

--
Vitaly