linux-kernel - Re: [PATCH] KVM: nVMX: Emulate NOPs in L2, and PAUSE if it's not intercepted

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Date:   Mon, 10 Apr 2023 16:31:05 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mathias Krause <minipli@...ecurity.net>
Subject: Re: [PATCH] KVM: nVMX: Emulate NOPs in L2, and PAUSE if it's not intercepted

On Tue, 04 Apr 2023 17:23:59 -0700, Sean Christopherson wrote:
> Extend VMX's nested intercept logic for emulated instructions to handle
> "pause" interception, in quotes because KVM's emulator doesn't filter out
> NOPs when checking for nested intercepts.  Failure to allow emulation of
> NOPs results in KVM injecting a #UD into L2 on any NOP that collides with
> the emulator's definition of PAUSE, i.e. on all single-byte NOPs.
> 
> For PAUSE itself, honor L1's PAUSE-exiting control, but ignore PLE to
> avoid unnecessarily injecting a #UD into L2.  Per the SDM, the first
> execution of PAUSE after VM-Entry is treated as the beginning of a new
> loop, i.e. will never trigger a PLE VM-Exit, and so L1 can't expect any
> given execution of PAUSE to deterministically exit.
> 
> [...]

Applied to kvm-x86 vmx.  I haven't gotten any reviews, but the FEP changes
in KUT will cause the nVMX test to fail, so I want to get kvm-x86/next fixed
sooner than later.  I'm not expecting anything else for "vmx", so unwinding
should be easy if it turns out this is busted/flawed.

[1/1] KVM: nVMX: Emulate NOPs in L2, and PAUSE if it's not intercepted
      https://github.com/kvm-x86/linux/commit/84f481315b10

--
https://github.com/kvm-x86/linux/tree/next
https://github.com/kvm-x86/linux/tree/fixes