Re: [PATCH v3 0/6] KVM: SVM: Fix DEBUGCTL bugs

[Sean Christopherson]

On Thu, 27 Feb 2025 14:24:05 -0800, Sean Christopherson wrote:
> Fix a long-lurking bug in SVM where KVM runs the guest with the host's
> DEBUGCTL if LBR virtualization is disabled.  AMD CPUs rather stupidly
> context switch DEBUGCTL if and only if LBR virtualization is enabled (not
> just supported, but fully enabled).
> 
> The bug has gone unnoticed because until recently, the only bits that
> KVM would leave set were things like BTF, which are guest visible but
> won't cause functional problems unless guest software is being especially
> particular about #DBs.
> 
> [...]

Applied 1-5 to kvm-x86 fixes (for 6.14).  I'm going to hold off on making
DEBUGCTL[5:2] reserved until at least 6.15.

[1/6] KVM: SVM: Drop DEBUGCTL[5:2] from guest's effective value
      https://github.com/kvm-x86/linux/commit/ee89e8013383
[2/6] KVM: SVM: Suppress DEBUGCTL.BTF on AMD
      https://github.com/kvm-x86/linux/commit/d0eac42f5cec
[3/6] KVM: x86: Snapshot the host's DEBUGCTL in common x86
      https://github.com/kvm-x86/linux/commit/fb71c7959356
[4/6] KVM: SVM: Manually context switch DEBUGCTL if LBR virtualization is disabled
      https://github.com/kvm-x86/linux/commit/433265870ab3
[5/6] KVM: x86: Snapshot the host's DEBUGCTL after disabling IRQs
      https://github.com/kvm-x86/linux/commit/189ecdb3e112
[6/6] KVM: SVM: Treat DEBUGCTL[5:2] as reserved
      (no commit info)

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