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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.03.2025 17:16, Alejandro Vallejo wrote:
> 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().
> 
> I prefer it that way too; I was simply trying to minimize the diff. Would you
> like me to resend it with that adjustment?

Yes please. Plus whatever else adjustments are needed in the subsequent patches
(sorry, it has been a while, so I don't recall the state of those later ones).

Jan