Re: [PATCH 3/4] i386/sev: Update checks and information related to reduced-phys-bits

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Tom Lendacky (thomas.lendacky@amd.com) wrote:
> The value of the reduced-phys-bits parameter is propogated to the CPUID
> information exposed to the guest. Update the current validation check to
> account for the size of the CPUID field (6-bits), ensuring the value is
> in the range of 1 to 63.
> 
> Maintain backward compatibility, to an extent, by allowing a value greater
> than 1 (so that the previously documented value of 5 still works), but not
> allowing anything over 63.
> 
> Fixes: d8575c6c02 ("sev/i386: add command to initialize the memory encryption context")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  target/i386/sev.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/target/i386/sev.c b/target/i386/sev.c
> index 32f7dbac4e..78c2d37eba 100644
> --- a/target/i386/sev.c
> +++ b/target/i386/sev.c
> @@ -932,15 +932,26 @@ int sev_kvm_init(ConfidentialGuestSupport *cgs, Error **errp)
>      host_cpuid(0x8000001F, 0, NULL, &ebx, NULL, NULL);
>      host_cbitpos = ebx & 0x3f;
>  
> +    /*
> +     * The cbitpos value will be placed in bit positions 5:0 of the EBX
> +     * register of CPUID 0x8000001F. No need to verify the range as the
> +     * comparison against the host value accomplishes that.
> +     */
>      if (host_cbitpos != sev->cbitpos) {
>          error_setg(errp, "%s: cbitpos check failed, host '%d' requested '%d'",
>                     __func__, host_cbitpos, sev->cbitpos);
>          goto err;
>      }
>  
> -    if (sev->reduced_phys_bits < 1) {
> -        error_setg(errp, "%s: reduced_phys_bits check failed, it should be >=1,"
> -                   " requested '%d'", __func__, sev->reduced_phys_bits);
> +    /*
> +     * The reduced-phys-bits value will be placed in bit positions 11:6 of
> +     * the EBX register of CPUID 0x8000001F, so verify the supplied value
> +     * is in the range of 1 to 63.
> +     */
> +    if (sev->reduced_phys_bits < 1 || sev->reduced_phys_bits > 63) {
> +        error_setg(errp, "%s: reduced_phys_bits check failed,"
> +                   " it should be in the range of 1 to 63, requested '%d'",
> +                   __func__, sev->reduced_phys_bits);
>          goto err;
>      }
>  
> -- 
> 2.37.3
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK