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

[Michael Larabel <Michael@MichaelLarabel.com>]

 From my testing this patch is in good shape for returning the EPYC 
Turin server bare metal performance w/o VMs back to where it was on 
Linux 6.14.

Thanks,

Michael

On 5/2/25 5:34 PM, 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.
>
> Similarly, don't bother optimizing the 1=>N and N=>1 transitions, e.g. by
> using atomic_inc_return() to avoid taking the spinlock, as ensuring that
> BP_SPEC_REDUCE is guaranteed to be set before KVM_RUN is non-trivial.  KVM
> already serializes VM creation against kvm_lock (to add the VM to vm_list),
> and the spinlock will only be held for a handful of cycles for the 1<=>N
> cases.  I.e. the complexity needed to ensure correctness outweighs the
> marginal benefits of eliding the lock.  See the Link for details.
>
> 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>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/svm/svm.c | 43 ++++++++++++++++++++++++++++++++++++------
>   1 file changed, 37 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index cc1c721ba067..364959fd1040 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -607,9 +607,6 @@ static void svm_disable_virtualization_cpu(void)
>   	kvm_cpu_svm_disable();
>   
>   	amd_pmu_disable_virt();
> -
> -	if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
> -		msr_clear_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);
>   }
>   
>   static int svm_enable_virtualization_cpu(void)
> @@ -687,9 +684,6 @@ static int svm_enable_virtualization_cpu(void)
>   		rdmsr(MSR_TSC_AUX, sev_es_host_save_area(sd)->tsc_aux, msr_hi);
>   	}
>   
> -	if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
> -		msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);
> -
>   	return 0;
>   }
>   
> @@ -5032,10 +5026,46 @@ static void svm_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>   	sev_vcpu_deliver_sipi_vector(vcpu, vector);
>   }
>   
> +#ifdef CONFIG_CPU_MITIGATIONS
> +static DEFINE_SPINLOCK(srso_lock);
> +static int srso_nr_vms;
> +
> +static void svm_toggle_srso_spec_reduce(void *set)
> +{
> +	if (set)
> +		msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);
> +	else
> +		msr_clear_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);
> +}
> +
> +static void svm_srso_add_remove_vm(int count)
> +{
> +	bool set;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
> +		return;
> +
> +	guard(spinlock)(&srso_lock);
> +
> +	set = !srso_nr_vms;
> +	srso_nr_vms += count;
> +
> +	WARN_ON_ONCE(srso_nr_vms < 0);
> +	if (!set && srso_nr_vms)
> +		return;
> +
> +	on_each_cpu(svm_toggle_srso_spec_reduce, (void *)set, 1);
> +}
> +#else
> +static void svm_srso_add_remove_vm(int count) { }
> +#endif
> +
>   static void svm_vm_destroy(struct kvm *kvm)
>   {
>   	avic_vm_destroy(kvm);
>   	sev_vm_destroy(kvm);
> +
> +	svm_srso_add_remove_vm(-1);
>   }
>   
>   static int svm_vm_init(struct kvm *kvm)
> @@ -5061,6 +5091,7 @@ static int svm_vm_init(struct kvm *kvm)
>   			return ret;
>   	}
>   
> +	svm_srso_add_remove_vm(1);
>   	return 0;
>   }
>   
>
> base-commit: 45eb29140e68ffe8e93a5471006858a018480a45