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From: Waiman Long <longman@redhat.com>
To: hpa@zytor.com, Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org, "Juergen Gross" <jgross@suse.com>,
	"Jeremy Fitzhardinge" <jeremy@goop.org>,
	x86@kernel.org, kvm@vger.kernel.org,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Pan Xinhui" <xinhui.pan@linux.vnet.ibm.com>,
	"Rusty Russell" <rusty@rustcorp.com.au>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	"Chris Wright" <chrisw@sous-sol.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	xen-devel@lists.xenproject.org,
	"Alok Kataria" <akataria@vmware.com>,
	"Thomas Gleixner" <tglx@linutronix.de>
Subject: Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function
Date: Mon, 13 Feb 2017 17:24:36 -0500	[thread overview]
Message-ID: <19008130-7b73-5c53-3cb5-a013e9e5552b__6651.40515561706$1487024747$gmane$org@redhat.com> (raw)
In-Reply-To: <F29A7066-16B9-4542-A686-292EC9CFF96E@zytor.com>

On 02/13/2017 03:06 PM, hpa@zytor.com wrote:
> On February 13, 2017 2:53:43 AM PST, Peter Zijlstra <peterz@infradead.org> wrote:
>> On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra wrote:
>>> That way we'd end up with something like:
>>>
>>> asm("
>>> push %rdi;
>>> movslq %edi, %rdi;
>>> movq __per_cpu_offset(,%rdi,8), %rax;
>>> cmpb $0, %[offset](%rax);
>>> setne %al;
>>> pop %rdi;
>>> " : : [offset] "i" (((unsigned long)&steal_time) + offsetof(struct
>> steal_time, preempted)));
>>> And if we could get rid of the sign extend on edi we could avoid all
>> the
>>> push-pop nonsense, but I'm not sure I see how to do that (then again,
>>> this asm foo isn't my strongest point).
>> Maybe:
>>
>> movsql %edi, %rax;
>> movq __per_cpu_offset(,%rax,8), %rax;
>> cmpb $0, %[offset](%rax);
>> setne %al;
>>
>> ?
> We could kill the zero or sign extend by changing the calling interface to pass an unsigned long instead of an int.  It is much more likely that a zero extend is free for the caller than a sign extend.

I have thought of that too. However, the goal is to eliminate memory
read/write from/to stack. Eliminating a register sign-extend instruction
won't help much in term of performance.

Cheers,
Longman


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  parent reply	other threads:[~2017-02-13 22:24 UTC|newest]

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-02-10 15:43 [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function Waiman Long
2017-02-10 15:43 ` Waiman Long
2017-02-10 16:19 ` Peter Zijlstra
2017-02-10 16:19 ` Peter Zijlstra
2017-02-10 16:19   ` Peter Zijlstra
2017-02-10 16:35   ` Waiman Long
2017-02-10 16:35     ` Waiman Long
2017-02-10 17:00     ` Waiman Long
2017-02-10 17:00     ` Waiman Long
2017-02-10 17:00       ` Waiman Long
2017-02-13 10:47       ` Peter Zijlstra
2017-02-13 10:47       ` Peter Zijlstra
2017-02-13 10:47         ` Peter Zijlstra
2017-02-13 10:53         ` Peter Zijlstra
2017-02-13 10:53         ` Peter Zijlstra
2017-02-13 10:53           ` Peter Zijlstra
2017-02-13 19:42           ` Waiman Long
2017-02-13 19:42           ` Waiman Long
2017-02-13 19:42           ` Waiman Long
2017-02-13 20:12             ` Waiman Long
2017-02-13 20:12               ` Waiman Long
2017-02-13 21:52               ` Peter Zijlstra
2017-02-13 21:52               ` Peter Zijlstra
2017-02-13 21:52                 ` Peter Zijlstra
2017-02-13 22:00                 ` hpa
2017-02-13 22:00                 ` hpa
2017-02-13 22:00                   ` hpa
2017-02-13 22:07                 ` hpa
2017-02-13 22:07                   ` hpa
2017-02-13 22:07                 ` hpa
2017-02-13 22:34                 ` Waiman Long
2017-02-13 22:34                 ` Waiman Long
2017-02-13 22:34                   ` Waiman Long
2017-02-13 22:36                   ` hpa
2017-02-13 22:36                     ` hpa
2017-02-13 22:36                   ` hpa
2017-02-14  9:39                   ` Peter Zijlstra
2017-02-14  9:39                   ` Peter Zijlstra
2017-02-14  9:39                     ` Peter Zijlstra
2017-02-14 14:46                     ` Waiman Long
2017-02-14 14:46                     ` Waiman Long
2017-02-14 14:46                       ` Waiman Long
2017-02-14 16:03                       ` Peter Zijlstra
2017-02-14 16:03                       ` Peter Zijlstra
2017-02-14 16:03                         ` Peter Zijlstra
2017-02-14 16:18                       ` [Xen-devel] " Andrew Cooper
2017-02-14 16:18                       ` Andrew Cooper
2017-02-14 16:18                       ` [Xen-devel] " Andrew Cooper
2017-02-13 20:12             ` Waiman Long
2017-02-13 20:06           ` hpa
2017-02-13 20:06             ` hpa
2017-02-13 21:57             ` Peter Zijlstra
2017-02-13 21:57               ` Peter Zijlstra
2017-02-13 21:57             ` Peter Zijlstra
2017-02-13 22:24             ` Waiman Long
2017-02-13 22:24               ` Waiman Long
2017-02-13 22:31               ` Peter Zijlstra
2017-02-13 22:31               ` Peter Zijlstra
2017-02-13 22:31                 ` Peter Zijlstra
2017-02-13 22:24             ` Waiman Long [this message]
2017-02-13 20:06           ` hpa
2017-02-13 19:41         ` Waiman Long
2017-02-13 19:41         ` Waiman Long
2017-02-13 19:41         ` Waiman Long
2017-02-10 16:35   ` Waiman Long
2017-02-10 16:22 ` Paolo Bonzini
2017-02-10 16:22 ` Paolo Bonzini
2017-02-10 16:22   ` Paolo Bonzini
  -- strict thread matches above, loose matches on Subject: below --
2017-02-10 15:43 Waiman Long

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