Re: [PATCH 6.6.y v1 6/6] KVM: SVM: Set/clear SRSO's BP_SPEC_REDUCE on 0 <=> 1 VM count transitions

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 29, 2026, Daniil Tatianin wrote:
> [ Upstream commit e3417ab75ab2e7dca6372a1bfa26b1be3ac5889e ]
> 
> 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.
> 
> Defer setting BP_SPEC_REDUCE until VMRUN is imminent to avoid impacting
> performance on CPUs that aren't running VMs, e.g. if a setup is using
> housekeeping CPUs.  Setting BP_SPEC_REDUCE in task context, i.e. without
> blasting IPIs to all CPUs, also helps avoid serializing 1<=>N transitions
> without incurring a gross amount of complexity (see the Link for details
> on how ugly coordinating via IPIs gets).
> 
> Link: https://lore.kernel.org/all/aBOnzNCngyS_pQIW@google.com
> Fixes: 8442df2b49ed ("x86/bugs: KVM: Add support for SRSO_MSR_FIX")
> Reported-by: Michael Larabel <Michael@michaellarabel.com>
> Closes: https://www.phoronix.com/review/linux-615-amd-regression
> Cc: Borislav Petkov <bp@alien8.de>
> Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
> Link: https://lore.kernel.org/r/20250505180300.973137-1-seanjc@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---

Acked-by: Sean Christopherson <seanjc@google.com>