Re: x86/bugs: KVM: Add support for SRSO_MSR_FIX, back for moar

Michael Larabel
2025-05-01T00:42:48Z

On 4/30/25 6:33 PM, Sean Christopherson wrote:
> On Tue, Apr 29, 2025, Borislav Petkov wrote:
>> On Tue, Feb 18, 2025 at 12:13:33PM +0100, Borislav Petkov wrote:
>>> So,
>>>
>>> in the interest of finally making some progress here I'd like to commit this
>>> below (will test it one more time just in case but it should work :-P). It is
>>> simple and straight-forward and doesn't need an IBPB when the bit gets
>>> cleared.
>>>
>>> A potential future improvement is David's suggestion that there could be a way
>>> for tracking when the first guest gets started, we set the bit then, we make
>>> sure the bit gets set on each logical CPU when the guests migrate across the
>>> machine and when the *last* guest exists, that bit gets cleared again.
>> Well, that "simplicity" was short-lived:
>>
>> https://www.phoronix.com/review/linux-615-amd-regression
> LOL.
>
>> Sean, how about this below?
> Eww. That's quite painful, and completely disallowing enable_virt_on_load is
> undesirable, e.g. for use cases where the host is (almost) exclusively running
> VMs.
>
> Best idea I have is to throw in the towel on getting fancy, and just maintain a
> dedicated count in SVM.
>
> Alternatively, we could plumb an arch hook into kvm_create_vm() and kvm_destroy_vm()
> that's called when KVM adds/deletes a VM from vm_list, and key off vm_list being
> empty. But that adds a lot of boilerplate just to avoid a mutex+count.
>
> I haven't tested on a system with X86_FEATURE_SRSO_BP_SPEC_REDUCE, but did verify
> the mechanics by inverting the flag.

Testing this patch on the same EPYC Turin server as my original tests, I
can confirm that on a clean boot without any VMs running, the
performance is back to where it was on v6.14. :)

Thanks,

Michael

>
> --
> From: Sean Christopherson <seanjc@google.com>
> Date: Wed, 30 Apr 2025 15:34:50 -0700
> Subject: [PATCH] KVM: SVM: Set/clear SRSO's BP_SPEC_REDUCE on 0 <=> 1 VM count
> transitions
>
> 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 mutex 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 sensity to VM creation
> latency.
>
> 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
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/svm/svm.c | 39 +++++++++++++++++++++++++++++++++------
> 1 file changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index d5d0c5c3300b..fe8866572218 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 +5026,42 @@ static void svm_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
> sev_vcpu_deliver_sipi_vector(vcpu, vector);
> }
>
> +static DEFINE_MUTEX(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(mutex)(&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);
> +}
> +
> 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 +5087,7 @@ static int svm_vm_init(struct kvm *kvm)
> return ret;
> }
>
> + svm_srso_add_remove_vm(1);
> return 0;
> }
>
>
> base-commit: f158e1b145f73aae1d3b7e756eb129a15b2b7a90
> --