RE: [RFC PATCH v1 4/6] x86/amd: Configure necessary MSRs for SNP during CPU init when running as a guest

["Michael Kelley (LINUX)" <mikelley@microsoft.com>]

From: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com> Sent: Monday, January 23, 2023 8:51 AM
> 
> Hyper-V may expose the SEV/SEV-SNP CPU features to the guest, but it is
> up to the guest to use them. early_detect_mem_encrypt() checks
> SYSCFG[MEM_ENCRYPT] and HWCR[SMMLOCK] and if these are not set the
> SEV-SNP features are cleared.  Check if we are running under a
> hypervisor and if so - update SYSCFG and skip the HWCR check.
> 
> It would be great to make this check more specific (checking for
> Hyper-V) but this code runs before hypervisor detection on the boot cpu.

Could you elaborate on why we would want this check to be Hyper-V
specific?   Per my comments on Patch 3 of this series, I would think the
opposite.  If possible, we want code like this to work on any hypervisor,
and not have Hyper-V specific behavior in code outside of the Hyper-V
modules.  But I don't know this code well at all, so maybe there's an
aspect I'm missing.

Michael

> 
> Signed-off-by: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
> ---
>  arch/x86/kernel/cpu/amd.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index c7884198ad5b..17d91ac62937 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -565,6 +565,12 @@ static void early_detect_mem_encrypt(struct cpuinfo_x86 *c)
>  	 *   don't advertise the feature under CONFIG_X86_32.
>  	 */
>  	if (cpu_has(c, X86_FEATURE_SME) || cpu_has(c, X86_FEATURE_SEV)) {
> +		if (cpu_has(c, X86_FEATURE_HYPERVISOR)) {
> +			rdmsrl(MSR_AMD64_SYSCFG, msr);
> +			msr |= MSR_AMD64_SYSCFG_MEM_ENCRYPT;
> +			wrmsrl(MSR_AMD64_SYSCFG, msr);
> +		}
> +
>  		/* Check if memory encryption is enabled */
>  		rdmsrl(MSR_AMD64_SYSCFG, msr);
>  		if (!(msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT))
> @@ -584,7 +590,7 @@ static void early_detect_mem_encrypt(struct cpuinfo_x86 *c)
>  			setup_clear_cpu_cap(X86_FEATURE_SME);
> 
>  		rdmsrl(MSR_K7_HWCR, msr);
> -		if (!(msr & MSR_K7_HWCR_SMMLOCK))
> +		if (!(msr & MSR_K7_HWCR_SMMLOCK) && !cpu_has(c, X86_FEATURE_HYPERVISOR))
>  			goto clear_sev;
> 
>  		return;
> --
> 2.25.1