Re: [PATCH] x86/sev: Check whether SEV or SME is supported first

[Tom Lendacky]

On 5/26/21 2:24 AM, Pu Wen wrote:
> The first two bits of the CPUID leaf 0x8000001F EAX indicate whether
> SEV or SME is supported respectively. It's better to check whether
> SEV or SME is supported before checking the SEV MSR(0xc0010131) to
> see whether SEV or SME is enabled.
> 
> This also avoid the MSR reading failure on the first generation Hygon
> Dhyana CPU which does not support SEV or SME.
> 
> Fixes: eab696d8e8b9 ("x86/sev: Do not require Hypervisor CPUID bit for SEV guests")
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+
> Signed-off-by: Pu Wen <puwen@xxxxxxxx>

I think the commit message needs to be expanded to clarify the situations
and provide more detail.

This is both a bare-metal issue and a guest/VM issue. Since Hygon doesn't
support the MSR_AMD64_SEV MSR, reading that MSR results in a #GP - either
directly from hardware in the bare-metal case or via the hypervisor
(because the RDMSR is actually intercepted) in the guest/VM case,
resulting in a failed boot. And since this is very early in the boot
phase, rdmsrl_safe()/native_read_msr_safe() can't be used.

So by checking the CPUID information before attempting the RDMSR, this
goes back to the behavior before the patch identified in the Fixes: tag.

With an improved commit message:

Acked-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>  arch/x86/mm/mem_encrypt_identity.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
> index a9639f663d25..470b20208430 100644
> --- a/arch/x86/mm/mem_encrypt_identity.c
> +++ b/arch/x86/mm/mem_encrypt_identity.c
> @@ -504,10 +504,6 @@ void __init sme_enable(struct boot_params *bp)
>  #define AMD_SME_BIT	BIT(0)
>  #define AMD_SEV_BIT	BIT(1)
>  
> -	/* Check the SEV MSR whether SEV or SME is enabled */
> -	sev_status   = __rdmsr(MSR_AMD64_SEV);
> -	feature_mask = (sev_status & MSR_AMD64_SEV_ENABLED) ? AMD_SEV_BIT : AMD_SME_BIT;
> -
>  	/*
>  	 * Check for the SME/SEV feature:
>  	 *   CPUID Fn8000_001F[EAX]
> @@ -519,11 +515,16 @@ void __init sme_enable(struct boot_params *bp)
>  	eax = 0x8000001f;
>  	ecx = 0;
>  	native_cpuid(&eax, &ebx, &ecx, &edx);
> -	if (!(eax & feature_mask))
> +	/* Check whether SEV or SME is supported */
> +	if (!(eax & (AMD_SEV_BIT | AMD_SME_BIT)))
>  		return;
>  
>  	me_mask = 1UL << (ebx & 0x3f);
>  
> +	/* Check the SEV MSR whether SEV or SME is enabled */
> +	sev_status   = __rdmsr(MSR_AMD64_SEV);
> +	feature_mask = (sev_status & MSR_AMD64_SEV_ENABLED) ? AMD_SEV_BIT : AMD_SME_BIT;
> +
>  	/* Check if memory encryption is enabled */
>  	if (feature_mask == AMD_SME_BIT) {
>  		/*
>