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

[Maxim Levitsky]

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@xxxxxxx>
> ---
>  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).

I am asking because there is a remote possibliity 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?

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

Best regards,
	Maxim Levitsky