Re: [PATCH 05/14] x86/xstate: Map/unmap xsave area in xstate_set_init() and handle_setbv()

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

On Tue Oct 29, 2024 at 8:26 AM GMT, Jan Beulich wrote:
> On 28.10.2024 16:49, Alejandro Vallejo wrote:
> > --- a/xen/arch/x86/xstate.c
> > +++ b/xen/arch/x86/xstate.c
> > @@ -993,7 +993,12 @@ int handle_xsetbv(u32 index, u64 new_bv)
> >  
> >          clts();
> >          if ( curr->fpu_dirtied )
> > -            asm ( "stmxcsr %0" : "=m" (curr->arch.xsave_area->fpu_sse.mxcsr) );
> > +        {
> > +            struct xsave_struct *xsave_area = vcpu_map_xsave_area(curr);
> > +
> > +            asm ( "stmxcsr %0" : "=m" (xsave_area->fpu_sse.mxcsr) );
> > +            vcpu_unmap_xsave_area(curr, xsave_area);
> > +        }
>
> Since it's curr that we're dealing with, is this largely a cosmetic change? I.e.
> there's no going to be any actual map/unmap operation in that case? Otherwise
> I'd be inclined to say that an actual map/unmap is pretty high overhead for a
> mere store of a 32-bit value.
>
> Jan

Somewhat.

See the follow-up reply to patch2 with something resembling what I expect the
wrappers to have. In short, yes, I expect "current" to not require
mapping/unmapping; but I still would rather see those sites using the same
wrappers for auditability. After we settle on a particular interface, we can
let the implementation details creep out if that happens to be clearer, but
it's IMO easier to work this way for the time being until those details
crystalise.

Cheers,
Alejandro