On 28/02/20 01:35, Oliver Upton wrote:
> The AVIC does not support guest use of the x2APIC interface. Currently,
> KVM simply chooses to squash the x2APIC feature in the guest's CPUID
> If the AVIC is enabled. Doing so prevents KVM from running a guest
> with greater than 255 vCPUs, as such a guest necessitates the use
> of the x2APIC interface.
>
> Instead, inhibit AVIC enablement on a per-VM basis whenever the x2APIC
> feature is set in the guest's CPUID. Since this changes the behavior of
> KVM as seen by userspace, add a module parameter, avic_per_vm, to opt-in
> for the new behavior. If this parameter is set, report x2APIC as
> available on KVM_GET_SUPPORTED_CPUID. Without opt-in, continue to
> suppress x2APIC from the guest's CPUID.
>
> Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
> Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
Since AVIC is not enabled by default, let's do this always and flip the
default to avic=1 in 5.7. People using avic=1 will have to disable
x2apic manually instead but the default will be the same in practice
(AVIC not enabled). And then we can figure out:
- how to do emulation of x2apic when avic is enabled (so it will cause
vmexits but still use the AVIC for e.g. assigned devices)
- a PV CPUID leaf to tell <=255 vCPUs on AMD virtualization _not_ to use
x2apic.
Paolo
> ---
>
> Parent commit: a93236fcbe1d ("KVM: s390: rstify new ioctls in api.rst")
>
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/svm.c | 19 ++++++++++++++++---
> 2 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 98959e8cd448..9d40132a3ae2 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -890,6 +890,7 @@ enum kvm_irqchip_mode {
> #define APICV_INHIBIT_REASON_NESTED 2
> #define APICV_INHIBIT_REASON_IRQWIN 3
> #define APICV_INHIBIT_REASON_PIT_REINJ 4
> +#define APICV_INHIBIT_REASON_X2APIC 5
>
> struct kvm_arch {
> unsigned long n_used_mmu_pages;
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index ad3f5b178a03..95c03c75f51a 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -382,6 +382,10 @@ module_param(sev, int, 0444);
> static bool __read_mostly dump_invalid_vmcb = 0;
> module_param(dump_invalid_vmcb, bool, 0644);
>
> +/* enable/disable opportunistic use of the AVIC on a per-VM basis */
> +static bool __read_mostly avic_per_vm;
> +module_param(avic_per_vm, bool, 0444);
> +
> static u8 rsm_ins_bytes[] = "\x0f\xaa";
>
> static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0);
> @@ -6027,7 +6031,15 @@ static void svm_cpuid_update(struct kvm_vcpu *vcpu)
> if (!kvm_vcpu_apicv_active(vcpu))
> return;
>
> - guest_cpuid_clear(vcpu, X86_FEATURE_X2APIC);
> + /*
> + * AVIC does not work with an x2APIC mode guest. If the X2APIC feature
> + * is exposed to the guest, disable AVIC.
> + */
> + if (avic_per_vm && guest_cpuid_has(vcpu, X86_FEATURE_X2APIC))
> + kvm_request_apicv_update(vcpu->kvm, false,
> + APICV_INHIBIT_REASON_X2APIC);
> + else
> + guest_cpuid_clear(vcpu, X86_FEATURE_X2APIC);
>
> /*
> * Currently, AVIC does not work with nested virtualization.
> @@ -6044,7 +6056,7 @@ static void svm_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
> {
> switch (func) {
> case 0x1:
> - if (avic)
> + if (avic && !avic_per_vm)
> entry->ecx &= ~F(X2APIC);
> break;
> case 0x80000001:
> @@ -7370,7 +7382,8 @@ static bool svm_check_apicv_inhibit_reasons(ulong bit)
> BIT(APICV_INHIBIT_REASON_HYPERV) |
> BIT(APICV_INHIBIT_REASON_NESTED) |
> BIT(APICV_INHIBIT_REASON_IRQWIN) |
> - BIT(APICV_INHIBIT_REASON_PIT_REINJ);
> + BIT(APICV_INHIBIT_REASON_PIT_REINJ) |
> + BIT(APICV_INHIBIT_REASON_X2APIC);
>
> return supported & BIT(bit);
> }
>
