Re: [PATCH] KVM: SVM: Do not virtualize MSR accesses for APIC LVTT register

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

Maxim,

On 7/28/22 2:38 PM, Maxim Levitsky wrote:
> On Sun, 2022-07-24 at 22:34 -0500, Suravee Suthikulpanit wrote:
>> AMD does not support APIC TSC-deadline timer mode. AVIC hardware
>> will generate GP fault when guest kernel writes 1 to bits [18]
>> of the APIC LVTT register (offset 0x32) to set the timer mode.
>> (Note: bit 18 is reserved on AMD system).
>>
>> Therefore, always intercept and let KVM emulate the MSR accesses.
>>
>> Fixes: f3d7c8aa6882 ("KVM: SVM: Fix x2APIC MSRs interception")
>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>> ---
>>   arch/x86/kvm/svm/svm.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index aef63aae922d..3e0639a68385 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -118,7 +118,14 @@ static const struct svm_direct_access_msrs {
>>   	{ .index = X2APIC_MSR(APIC_ESR),		.always = false },
>>   	{ .index = X2APIC_MSR(APIC_ICR),		.always = false },
>>   	{ .index = X2APIC_MSR(APIC_ICR2),		.always = false },
>> -	{ .index = X2APIC_MSR(APIC_LVTT),		.always = false },
>> +
>> +	/*
>> +	 * Note:
>> +	 * AMD does not virtualize APIC TSC-deadline timer mode, but it is
>> +	 * emulated by KVM. When setting APIC LVTT (0x832) register bit 18,
>> +	 * the AVIC hardware would generate GP fault. Therefore, always
>> +	 * intercept the MSR 0x832, and do not setup direct_access_msr.
>> +	 */
>>   	{ .index = X2APIC_MSR(APIC_LVTTHMR),		.always = false },
>>   	{ .index = X2APIC_MSR(APIC_LVTPC),		.always = false },
>>   	{ .index = X2APIC_MSR(APIC_LVT0),		.always = false },
> 
> 
> LVT is not something I would expect x2avic to even try to emulate, I would expect
> it to dumbly forward the write to apic backing page (garbage in, garbage out) and then
> signal trap vmexit?
> 
> I also think that regular AVIC works like that (just forwards the write to the page).

The main difference b/w AVIC and x2AVIC is the MSR interception control, which needs to
not-intercept x2APIC MSRs for x2AVIC (allowing HW to virtualize MSR accesses).
However, the hypervisor can decide which x2APIC MSR to intercept and emulate.

> I am asking because there is a remote possibility that due to some bug the guest got
> direct access to x2apic registers of the host, and this is how you got that #GP.
> Could you double check it?

I have verified this behavior with the HW designer and requested them to document
this in the next AMD programmers manual that will include x2AVIC details.

> We really need x2avic (and vNMI) spec to be published to know exactly how all of this
> is supposed to work.

I have raised the concern to the team responsible for publishing the doc.

Best Regards,
Suravee