Re: [PATCH v2] KVM: SEV-ES: Don't intercept MSR_IA32_DEBUGCTLMSR for SEV-ES guests

["Gupta, Pankaj" <pankaj.gupta@xxxxxxx>]

On 4/16/2024 7:03 AM, Ravi Bangoria wrote:
> Currently, LBR Virtualization is dynamically enabled and disabled for
> a vcpu by intercepting writes to MSR_IA32_DEBUGCTLMSR. This helps by
> avoiding unnecessary save/restore of LBR MSRs when nobody is using it
> in the guest. However, SEV-ES guest mandates LBR Virtualization to be
> _always_ ON[1] and thus this dynamic toggling doesn't work for SEV-ES
> guest, in fact it results into fatal error:
>
> SEV-ES guest on Zen3, kvm-amd.ko loaded with lbrv=1
>
>    [guest ~]# wrmsr 0x1d9 0x4
>    KVM: entry failed, hardware error 0xffffffff
>    EAX=00000004 EBX=00000000 ECX=000001d9 EDX=00000000
>    ...
>
> Fix this by never intercepting MSR_IA32_DEBUGCTLMSR for SEV-ES guests.
> No additional save/restore logic is required since MSR_IA32_DEBUGCTLMSR
> is of swap type A.
>
> [1]: AMD64 Architecture Programmer's Manual Pub. 40332, Rev. 4.07 - June
>       2023, Vol 2, 15.35.2 Enabling SEV-ES.
>       https://bugzilla.kernel.org/attachment.cgi?id=304653
>
> Fixes: 376c6d285017 ("KVM: SVM: Provide support for SEV-ES vCPU creation/loading")
> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
> Reviewed-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

Reviewed-by: Pankaj Gupta <pankaj.gupta@xxxxxxx>

> ---
> v1: https://lore.kernel.org/r/20240326081143.715-1-ravi.bangoria@xxxxxxx
> v1->v2:
>    - Add MSR swap type detail in the patch description. No code changes.
>
>   arch/x86/kvm/svm/sev.c | 1 +
>   arch/x86/kvm/svm/svm.c | 1 +
>   arch/x86/kvm/svm/svm.h | 2 +-
>   3 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index a8ce5226b3b5..ef932a7ff9bd 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3073,6 +3073,7 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
>   	/* Clear intercepts on selected MSRs */
>   	set_msr_interception(vcpu, svm->msrpm, MSR_EFER, 1, 1);
>   	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_CR_PAT, 1, 1);
> +	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_DEBUGCTLMSR, 1, 1);
>   	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHFROMIP, 1, 1);
>   	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHTOIP, 1, 1);
>   	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTINTFROMIP, 1, 1);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index e90b429c84f1..5a82135ae84e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -99,6 +99,7 @@ static const struct svm_direct_access_msrs {
>   	{ .index = MSR_IA32_SPEC_CTRL,			.always = false },
>   	{ .index = MSR_IA32_PRED_CMD,			.always = false },
>   	{ .index = MSR_IA32_FLUSH_CMD,			.always = false },
> +	{ .index = MSR_IA32_DEBUGCTLMSR,		.always = false },
>   	{ .index = MSR_IA32_LASTBRANCHFROMIP,		.always = false },
>   	{ .index = MSR_IA32_LASTBRANCHTOIP,		.always = false },
>   	{ .index = MSR_IA32_LASTINTFROMIP,		.always = false },
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 8ef95139cd24..7a1b60bcebff 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -30,7 +30,7 @@
>   #define	IOPM_SIZE PAGE_SIZE * 3
>   #define	MSRPM_SIZE PAGE_SIZE * 2
>   
> -#define MAX_DIRECT_ACCESS_MSRS	47
> +#define MAX_DIRECT_ACCESS_MSRS	48
>   #define MSRPM_OFFSETS	32
>   extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
>   extern bool npt_enabled;