Commit e3417ab7 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

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 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: 8442df2b ("x86/bugs: KVM: Add support for SRSO_MSR_FIX")
Reported-by: default avatarMichael Larabel <Michael@michaellarabel.com>
Closes: https://www.phoronix.com/review/linux-615-amd-regression


Cc: Borislav Petkov <bp@alien8.de>
Tested-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250505180300.973137-1-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 9129633d

arch/x86/kvm/svm/svm.c

+65 −6
Original line number Diff line number Diff line
@@ -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;

}
@@ -1518,6 +1512,63 @@ static void svm_vcpu_free(struct kvm_vcpu *vcpu) 
	__free_pages(virt_to_page(svm->msrpm), get_order(MSRPM_SIZE));

}



#ifdef CONFIG_CPU_MITIGATIONS

static DEFINE_SPINLOCK(srso_lock);

static atomic_t srso_nr_vms;



static void svm_srso_clear_bp_spec_reduce(void *ign)

{

	struct svm_cpu_data *sd = this_cpu_ptr(&svm_data);



	if (!sd->bp_spec_reduce_set)

		return;



	msr_clear_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);

	sd->bp_spec_reduce_set = false;

}



static void svm_srso_vm_destroy(void)

{

	if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))

		return;



	if (atomic_dec_return(&srso_nr_vms))

		return;



	guard(spinlock)(&srso_lock);



	/*

	 * Verify a new VM didn't come along, acquire the lock, and increment

	 * the count before this task acquired the lock.

	 */

	if (atomic_read(&srso_nr_vms))

		return;



	on_each_cpu(svm_srso_clear_bp_spec_reduce, NULL, 1);

}



static void svm_srso_vm_init(void)

{

	if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))

		return;



	/*

	 * Acquire the lock on 0 => 1 transitions to ensure a potential 1 => 0

	 * transition, i.e. destroying the last VM, is fully complete, e.g. so

	 * that a delayed IPI doesn't clear BP_SPEC_REDUCE after a vCPU runs.

	 */

	if (atomic_inc_not_zero(&srso_nr_vms))

		return;



	guard(spinlock)(&srso_lock);



	atomic_inc(&srso_nr_vms);

}

#else

static void svm_srso_vm_init(void) { }

static void svm_srso_vm_destroy(void) { }

#endif



static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)

{

	struct vcpu_svm *svm = to_svm(vcpu);
@@ -1550,6 +1601,11 @@ static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu) 
	    (!boot_cpu_has(X86_FEATURE_V_TSC_AUX) || !sev_es_guest(vcpu->kvm)))

		kvm_set_user_return_msr(tsc_aux_uret_slot, svm->tsc_aux, -1ull);



	if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE) &&

	    !sd->bp_spec_reduce_set) {

		sd->bp_spec_reduce_set = true;

		msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);

	}

	svm->guest_state_loaded = true;

}
@@ -5040,6 +5096,8 @@ static void svm_vm_destroy(struct kvm *kvm) 
{

	avic_vm_destroy(kvm);

	sev_vm_destroy(kvm);



	svm_srso_vm_destroy();

}



static int svm_vm_init(struct kvm *kvm)
@@ -5065,6 +5123,7 @@ static int svm_vm_init(struct kvm *kvm) 
			return ret;

	}



	svm_srso_vm_init();

	return 0;

}

arch/x86/kvm/svm/svm.h

+2 −0
Original line number Diff line number Diff line
@@ -335,6 +335,8 @@ struct svm_cpu_data { 
	u32 next_asid;

	u32 min_asid;



	bool bp_spec_reduce_set;



	struct vmcb *save_area;

	unsigned long save_area_pa;