Re: [PATCH v2 09/13] x86/emulator: Refactor FXSAVE_AREA to use wrappers

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

On Mon Dec 9, 2024 at 4:26 PM GMT, Jan Beulich wrote:
> On 05.11.2024 15:33, 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(currt ent, x)
>
> The typo of the first argument strongly suggests that the macro should
> already now evaluate its parameters, also pleasing Misra.

Not an unreasonable takeaway. I can expand as follows instead:

#define VCPU_UNMAP_XSAVE_AREA(v, x) ({ (void)(v); x; })

Thoughts?

>
> >  # else
> >  #  define FXSAVE_AREA get_fpu_save_area()
> > +#  define UNMAP_FXSAVE_AREA(x) ((void)x)
>
> If only for consistency and to avoid setting bad precedents - parentheses
> please around x.

Ack.

>
> > @@ -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;
> >      }
>
> So for now the emulator only supports FXSAVE / FXRSTOR. That'll need to change
> sooner or later. Is it really appropriate in that light to name the new macro
> after FXSAVE, when the underlying machinery uses all XSAVE?
>
> Jan

I have no strong feeling one way or the other, except that it should mirror the
other macro's name. I'd say it makes more sense to rename _both_ after the
emulator is XSAVE-aware. That the internal machinery is actually XSAVE-based is
an implementation detail largely irrelevant at the call sites.

Cheers,
Alejandro