Re: [Qemu-devel] [PATCH v1 14/33] s390x/tcg: Implement VECTOR LOAD MULTIPLE

[David Hildenbrand <david@redhat.com>]

On 27.02.19 17:02, Richard Henderson wrote:
> On 2/26/19 3:38 AM, David Hildenbrand wrote:
>> Also fairly easy to implement. One issue we have is that exceptions will
>> result in some vectors already being modified. At least handle it
>> consistently per vector by using a temporary vector. Good enough for
>> now, add a FIXME.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  target/s390x/insn-data.def      |  2 ++
>>  target/s390x/translate_vx.inc.c | 26 ++++++++++++++++++++++++++
>>  2 files changed, 28 insertions(+)
> 
> I suppose the fixme is good enough.  For the record, I think you could do the
> check with just two loads -- the first and last quadword.  After that, none of
> the other loads can fault, and you can store everything else into the
> destination vectors as you read them.

Aren't such approaches prone to races if other VCPUs invalidate page
tables/TLB entries?

(or am I messing up things and the MMU of this VCPU won't be touched
while in this block and once we touched all applicable pages, it cannot
fail anymore?)

> 
> Also missing for the fixme: MO_ALIGN{,_16}.

Just like the other occurrence, I think MO_ALIGN would be wrong.

"Setting the alignment hint to a non-zero value
that doesn’t correspond to the alignment of the
second operand may reduce performance on
some models."

> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> 

Thanks!

> 
> r~
> 


-- 

Thanks,

David / dhildenb