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

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v1 14/33] s390x/tcg: Implement VECTOR LOAD MULTIPLE
Date: Thu, 28 Feb 2019 09:15:52 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 2/28/19 12:36 AM, David Hildenbrand wrote:
> 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 <address@hidden>
>>> ---
>>>  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?

No, because...

> (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?)


If vcpu 1 does a global invalidate, the time at which vcpu 2 acknowledges that
invalidate is somewhat fluid.  VCPU 2 will see an interrupt, exit at a TB
boundary, and then acknowledge.

VCPU 1 has to wait for the ack before it knows the operation is complete.

Thus no race within any given instruction's execution.


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