Re: [RFC PATCH] pvh: Introduce SIF_HVM_GHCB for SEV-ES/SNP guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.12.2025 13:49, Teddy Astie wrote:
> Under SEV, the pagetables needs to be post-processed to add the C-bit
> (to make the mapping encrypted). The guest is expected to query the C-bit
> through CPUID. However, under SEV-ES and SEV-SNP modes, this instruction
> now triggers #VC instead. The guest would need to setup a IDT very early
> and instead use the early-GHCB protocol to emulate CPUID, which is
> complicated.

But isn't this going to be needed for plain HVM anyway?

> --- a/xen/include/public/xen.h
> +++ b/xen/include/public/xen.h
> @@ -890,6 +890,8 @@ typedef struct start_info start_info_t;
>  #define SIF_MOD_START_PFN (1<<3)  /* Is mod_start a PFN? */
>  #define SIF_VIRT_P2M_4TOOLS (1<<4) /* Do Xen tools understand a virt. mapped 
> */
>                                     /* P->M making the 3 level tree obsolete? 
> */
> +#define SIF_HVM_GHCB      (1<<5)   /* Domain is SEV-ES/SNP guest that 
> requires */
> +                                   /* use of GHCB. */

Naming-wise, do we really want to tie this to AMD (and hence exclude other
vendors, or require yet another bit to be allocated later)?

Jan