On Fri, 2024-11-01 at 11:35 -0700, Sean Christopherson wrote:
> Do nothing in from kvm_apic_set_base() if the incoming MSR value is the
> same as the current value, as validating the mode transitions is obviously
> unnecessary, and rejecting the write is pointless if the vCPU already has
> an invalid value, e.g. if userspace is doing weird things and modified
> guest CPUID after setting MSR_IA32_APICBASE.
>
> Bailing early avoids kvm_recalculate_apic_map()'s slow path in the rare
> scenario where the map is DIRTY due to some other vCPU dirtying the map,
> in which case it's the other vCPU/task's responsibility to recalculate the
> map.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/lapic.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 7b2342e40e4e..59a64b703aad 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2582,9 +2582,6 @@ static void __kvm_apic_set_base(struct kvm_vcpu *vcpu, u64 value)
> u64 old_value = vcpu->arch.apic_base;
> struct kvm_lapic *apic = vcpu->arch.apic;
>
> - if (old_value == value)
> - return;
> -
Could you clarify why this is removed? AFAICT kvm_lapic_reset() still calls
directly.
> vcpu->arch.apic_base = value;
>
> if ((old_value ^ value) & MSR_IA32_APICBASE_ENABLE)
> @@ -2632,6 +2629,10 @@ int kvm_apic_set_base(struct kvm_vcpu *vcpu, u64 value, bool host_initiated)
> {
> enum lapic_mode old_mode = kvm_get_apic_mode(vcpu);
> enum lapic_mode new_mode = kvm_apic_mode(value);
> +
> + if (vcpu->arch.apic_base == value)
> + return 0;
> +
> u64 reserved_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu) | 0x2ff |
> (guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) ? 0 : X2APIC_ENABLE);
>
> --
> 2.47.0.163.g1226f6d8fa-goog
>
