From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [GIT PULL 04/23] KVM: s390: rewrite vcpu_post_run and drop out
 early
Date: Wed, 2 Dec 2015 15:00:01 +0100
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: KVM <kvm@vger.kernel.org>, linux-s390 <linux-s390@vger.kernel.org>, Cornelia Huck <cornelia.huck@de.ibm.com>, Jens Freimann <jfrei@linux.vnet.ibm.com>, Alexander Graf <agraf@suse.de>
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On 12/02/2015 02:05 PM, Paolo Bonzini wrote:
> 
> 
> On 02/12/2015 14:04, Christian Borntraeger wrote:
>>>> Do you gain much over the simpler
>>>>
>>>> 	vcpu->run->s.regs.gprs[14] = vcpu->arch.sie_block->gg14;
>>>> 	vcpu->run->s.regs.gprs[15] = vcpu->arch.sie_block->gg15;
>>>>
>> Its just legacy code from the old days.
>> There is a difference, but it seems to a missed opportunity from gcc
>>
>>         vcpu->arch.sie_block->gg14 = vcpu->run->s.regs.gprs[14];
>>     839c:       e3 30 f0 b8 00 04       lg      %r3,184(%r15)
>>     83a2:       e3 10 32 40 00 04       lg      %r1,576(%r3)
>>     83a8:       e3 20 30 80 00 04       lg      %r2,128(%r3)
>>     83ae:       e3 20 21 b8 00 04       lg      %r2,440(%r2)
>>     83b4:       e3 20 10 a0 00 24       stg     %r2,160(%r1)
>>         vcpu->arch.sie_block->gg15 = vcpu->run->s.regs.gprs[15];
>>     83ba:       e3 10 32 40 00 04       lg      %r1,576(%r3)
>>     83c0:       e3 20 30 80 00 04       lg      %r2,128(%r3)
>>     83c6:       e3 20 21 c0 00 04       lg      %r2,448(%r2)
>>     83cc:       e3 20 10 a8 00 24       stg     %r2,168(%r1)
>>
>> gcc seems to reuse and reload %r2 and %r3, maybe register pressure.
> 
> More likely to be -fno-strict-aliasing. :(

Yes its indeed the aliasing.