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

["Suthikulpanit, Suravee" <suravee.suthikulpanit@amd.com>]

Sean,

On 7/19/2022 12:28 AM, Sean Christopherson wrote:
> On Mon, Jul 18, 2022, 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")
> 
> This tag is wrong, I believe it should be:
> 
>    Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
> 
> And that absolutely matters because this should not be backported to older
> kernels that don't support x2avic.

The commit 5413bcba7ed5 is the one that modifies the logic in the kvm_apic_write_nodecode().
I understand your point that the 5413bcba7ed is committed later than 4d1d7942e36a and being
affected by the change. However, if there is a case that only x2AVIC stuff is being backported
w/o the virtualize IPI stuff, then this fix is not needed. Hence, I would say the fix is for
the 5413bcba7ed5 as specified in the original patch.

>> .....
>>   		 */
>> -		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);
> 
> Because this lacks the TODO below, what about tweaking this so that there's a
> single call to kvm_lapic_msr_write()?  gcc-11 even generates more efficient code
> for this.  Alternatively, the ICR path can be an early return inside a single
> x2APIC check, but gcc generate identical code and I like making x2APIC+ICR stand
> out as being truly special.

That sounds good.

> Compile tested only.
> 
> ---
> From: Sean Christopherson <seanjc@google.com>
> Date: Mon, 18 Jul 2022 10:16:02 -0700
> Subject: [PATCH] KVM: x86: Handle trap-like x2APIC accesses for any APIC
>   register
> 
> Handle trap-like VM-Exits for all APIC registers when the guest is in
> x2APIC mode and drop the now-stale WARN that KVM encounters trap-like
> exits only for ICR.  On Intel, only writes to ICR can be trap-like when
> APICv and x2APIC are enabled, but AMD's x2AVIC can trap more registers,
> e.g. LDR and DFR.
> 
> Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
> Reported-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> Cc: Zeng Guang <guang.zeng@intel.com>
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/lapic.c | 21 ++++++++++-----------
>   1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9d4f73c4dc02..95bb1ef37a12 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2283,21 +2283,20 @@ void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset)
>   	struct kvm_lapic *apic = vcpu->arch.apic;
>   	u64 val;
> 
> -	if (apic_x2apic_mode(apic)) {
> -		/*
> -		 * 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.
> -		 */
> -		if (WARN_ON_ONCE(offset != APIC_ICR))
> -			return;
> -
> +	if (apic_x2apic_mode(apic))
>   		kvm_lapic_msr_read(apic, offset, &val);
> +	else
> +		val = kvm_lapic_get_reg(apic, offset);
> +
> +	/*
> +	 * ICR is a single 64-bit register when x2APIC is enabled.  For legacy
> +	 * xAPIC, ICR writes need to go down the common (slightly slower) path
> +	 * to get the upper half from ICR2.
> +	 */
> +	if (apic_x2apic_mode(apic) && offset == APIC_ICR) {
>   		kvm_apic_send_ipi(apic, (u32)val, (u32)(val >> 32));
>   		trace_kvm_apic_write(APIC_ICR, val);
>   	} else {
> -		val = kvm_lapic_get_reg(apic, offset);
> -
>   		/* TODO: optimize to just emulate side effect w/o one more write */
>   		kvm_lapic_reg_write(apic, offset, (u32)val);
>   	}
> 
> base-commit: 8031d87aa9953ddeb047a5356ebd0b240c30f233
> --

Tested-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>

Suravee