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

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

On Wed Mar 5, 2025 at 3:29 PM GMT, Jan Beulich wrote:
> 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 ...

Thanks for coming back to it :)

>
> > @@ -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

I prefer it that way too; I was simply trying to minimize the diff. Would you
like me to resend it with that adjustment?

Cheers,
Alejandro