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

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Message-ID: <87leqcoglp.fsf@redhat.com>
Date:   Wed, 21 Sep 2022 18:08:18 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v4 1/9] KVM: x86/mmu: Bug the VM if KVM attempts to
 double count an NX huge page

Sean Christopherson <seanjc@...gle.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