Re: [PATCH v2] KVM: SVM: Set/clear SRSO's BP_SPEC_REDUCE on 0 <=> 1 VM count transitions

[Sean Christopherson]

On Mon, 05 May 2025 11:03:00 -0700, Sean Christopherson wrote:
> Set the magic BP_SPEC_REDUCE bit to mitigate SRSO when running VMs if and
> only if KVM has at least one active VM.  Leaving the bit set at all times
> unfortunately degrades performance by a wee bit more than expected.
> 
> Use a dedicated spinlock and counter instead of hooking virtualization
> enablement, as changing the behavior of kvm.enable_virt_at_load based on
> SRSO_BP_SPEC_REDUCE is painful, and has its own drawbacks, e.g. could
> result in performance issues for flows that are sensitive to VM creation
> latency.
> 
> [...]

Applied to kvm-x86 fixes.  Assuming -next doesn't explode overnight, I'll get
a pull request sent to Paolo tomorrow.

[1/1] KVM: SVM: Set/clear SRSO's BP_SPEC_REDUCE on 0 <=> 1 VM count transitions
      https://github.com/kvm-x86/linux/commit/e3417ab75ab2

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