From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH] x86: compact supposedly unused entry point code
Date: Mon, 20 Jun 2016 07:49:18 -0600 [thread overview]
Message-ID: <5768107E02000078000F6DA5@prv-mh.provo.novell.com> (raw)
In-Reply-To: <6cccba46-2a7d-9369-a340-57518685da50@citrix.com>
>>> On 20.06.16 at 14:54, <andrew.cooper3@citrix.com> wrote:
> On 20/06/16 13:48, Jan Beulich wrote:
>>>>> On 20.06.16 at 14:15, <andrew.cooper3@citrix.com> wrote:
>>> On 20/06/16 12:04, Jan Beulich wrote:
>>>> No point in aligning entry points which aren't supposed to be used
>>>> anyway.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> Thanks, but - any thoughts on this part:
>>
>> TBD: Might consider simply using "andq $-15,%rsp", delivering an
>> uninitialized error code (which shouldn't matter).
>
> I was still considering that part.
>
> These are entries we never expect to actually take. At that point, the
> small overhead of setting up the error code to 0 is probably better than
> leaving it uninitialised.
I understand - it's really a matter of balancing the overhead on
these paths (which will never have an effect if these entries indeed
are unused, and which is of no interest if they are used by due some
other flaw) with the (likely negligible, but non-zero) overhead they
introduce on _other_ paths (due to cache and TLB consumption). I.e.
my goal was to make these unused entries as small as possible. And
andq $-15,%rsp
movl $vector,4(%rsp)
(obviously we can't use movb here) is smaller than the current
testb $8,%spl
jz 1f
pushq $0
movb $vector,4(%rsp)
afaict.
Jan
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next prev parent reply other threads:[~2016-06-20 13:49 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-20 11:04 [PATCH] x86: compact supposedly unused entry point code Jan Beulich
2016-06-20 12:15 ` Andrew Cooper
2016-06-20 12:48 ` Jan Beulich
2016-06-20 12:54 ` Andrew Cooper
2016-06-20 13:49 ` Jan Beulich [this message]
2016-06-20 13:58 ` Andrew Cooper
2016-06-20 14:04 ` Jan Beulich
2016-06-21 17:47 ` Andrew Cooper
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