Re: [PATCH v3 08/12] x86/emulator: Refactor FXSAVE_AREA to use wrappers

[Jan Beulich]

On 10.01.2025 14:28, Alejandro Vallejo wrote:
> --- a/xen/arch/x86/x86_emulate/blk.c
> +++ b/xen/arch/x86/x86_emulate/blk.c
> @@ -11,9 +11,12 @@
>      !defined(X86EMUL_NO_SIMD)
>  # ifdef __XEN__
>  #  include <asm/xstate.h>
> -#  define FXSAVE_AREA ((void *)&current->arch.xsave_area->fpu_sse)
> +/* Has a fastpath for `current`, so there's no actual map */
> +#  define FXSAVE_AREA ((void *)VCPU_MAP_XSAVE_AREA(current))
> +#  define UNMAP_FXSAVE_AREA(x) VCPU_UNMAP_XSAVE_AREA(current, x)
>  # else
>  #  define FXSAVE_AREA get_fpu_save_area()
> +#  define UNMAP_FXSAVE_AREA(x) ((void)(x))
>  # endif
>  #endif

While preparing to commit this I felt a little uneasy. The mapping aspect
is ...

> @@ -292,6 +295,9 @@ int x86_emul_blk(
>          }
>          else
>              asm volatile ( "fxrstor %0" :: "m" (*fxsr) );
> +
> +        UNMAP_FXSAVE_AREA(fxsr);
> +
>          break;
>      }
>  
> @@ -320,6 +326,9 @@ int x86_emul_blk(
>  
>          if ( fxsr != ptr ) /* i.e. s->op_bytes < sizeof(*fxsr) */
>              memcpy(ptr, fxsr, s->op_bytes);
> +
> +        UNMAP_FXSAVE_AREA(fxsr);
> +
>          break;
>      }
>  

... is entirely invisible at the use sites. This imo calls for making
mistakes, and hence the existing macro better would be adjusted to become
MAP_FXSAVE_AREA().

Jan