Re: [PATCH] KVM: x86: Do not block APIC write for non ICR registers

[Maxim Levitsky]

On Mon, 2022-07-18 at 03:39 -0500, Suravee Suthikulpanit wrote:
> The commit 5413bcba7ed5 ("KVM: x86: Add support for vICR APIC-write
> VM-Exits in x2APIC mode") introduces logic to prevent APIC write
> for offset other than ICR. This breaks x2AVIC support, which requires
> KVM to trap and emulate x2APIC MSR writes.
> 
> Therefore, removes the warning and modify to logic to allow MSR write.
> 
> Fixes: 5413bcba7ed5 ("KVM: x86: Add support for vICR APIC-write VM-Exits in x2APIC mode")
> Cc: Zeng Guang <guang.zeng@xxxxxxxxx>
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> ---
>  arch/x86/kvm/lapic.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9d4f73c4dc02..f688090d98b0 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -69,6 +69,7 @@ static bool lapic_timer_advance_dynamic __read_mostly;
>  /* step-by-step approximation to mitigate fluctuation */
>  #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
>  static int kvm_lapic_msr_read(struct kvm_lapic *apic, u32 reg, u64 *data);
> +static int kvm_lapic_msr_write(struct kvm_lapic *apic, u32 reg, u64 data);
>  
>  static inline void __kvm_lapic_set_reg(char *regs, int reg_off, u32 val)
>  {
> @@ -2284,17 +2285,23 @@ void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset)
>         u64 val;
>  
>         if (apic_x2apic_mode(apic)) {
> +               kvm_lapic_msr_read(apic, offset, &val);
> +
>                 /*
>                  * When guest APIC is in x2APIC mode and IPI virtualization
>                  * is enabled, accessing APIC_ICR may cause trap-like VM-exit
>                  * on Intel hardware. Other offsets are not possible.
> +                *
> +                * For AMD AVIC, write to some APIC registers can cause
> +                * trap-like VM-exit (see arch/x86/kvm/svm/avic.c:
> +                * avic_unaccel_trap_write()).
>                  */
> -               if (WARN_ON_ONCE(offset != APIC_ICR))
> +               if (offset == APIC_ICR) {
> +                       kvm_apic_send_ipi(apic, (u32)val, (u32)(val >> 32));
> +                       trace_kvm_apic_write(APIC_ICR, val);
>                         return;
> -
> -               kvm_lapic_msr_read(apic, offset, &val);
> -               kvm_apic_send_ipi(apic, (u32)val, (u32)(val >> 32));
> -               trace_kvm_apic_write(APIC_ICR, val);
> +               }
> +               kvm_lapic_msr_write(apic, offset, val);
>         } else {
>                 val = kvm_lapic_get_reg(apic, offset);
>  


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky