Re: [PATCH v3 3/4] xen/hvm: introduce a fpu_uninitialised field to the CPU save record

["Roger Pau Monné" <roger.pau@citrix.com>]

El 20/11/15 a les 16.49, Jan Beulich ha escrit:
>>>> On 18.11.15 at 17:37, <roger.pau@citrix.com> wrote:
>> @@ -2091,7 +2092,8 @@ static int hvm_load_cpu_ctxt(struct domain *d, hvm_domain_context_t *h)
>>          struct xsave_struct *xsave_area = v->arch.xsave_area;
>>  
>>          memcpy(v->arch.xsave_area, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
>> -        xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
>> +        xsave_area->xsave_hdr.xstate_bv = ctxt.fpu_initialised ?
>> +                                                    XSTATE_FP_SSE : 0;
>>      }
>>      else
>>          memcpy(v->arch.fpu_ctxt, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
> 
> Question is - are the memcpy()s here really meaningful/valid
> when !ctxt.fpu_initialized? I.e. shouldn't this code rather be
> skipped instead of getting modified?

If !fpu_initialized the fpu context save record is all zeroed out. I
don't think it matters much (apart from saving a few CPU cycles). I can
send a new version that doesn't save/restore the fpu context at all if
!fpu_initialised.

>> @@ -157,6 +159,8 @@ struct hvm_hw_cpu {
>>      };
>>      /* error code for pending event */
>>      uint32_t error_code;
>> +    /* is fpu initialised? */
>> +    uint32_t fpu_initialised;
> 
> A whole uint32_t for just one bit? Didn't we talk about making this
> new field a flags one, consuming just one bit from it?

AFAIK we agreed on adding this field to the tail and making it a
uint32_t so that when new fields are added they can be detected by
looking at the size of the structure:

http://marc.info/?l=xen-devel&m=144490321208291

>> @@ -266,6 +270,7 @@ struct hvm_hw_cpu_compat {
>>      };
>>      /* error code for pending event */
>>      uint32_t error_code;
>> +    /*uint32_t fpu_initialised; COMPAT */
>>  };
> 
> I think this is misleading - the compat structure never has this field.

Right, I will remove it.

Roger.