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. -- From: Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> Closes: https://www.phoronix.com/review/linux-615-amd-regression Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- 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 --
