Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function

From: Waiman Long <longman@redhat.com>
To: 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>,
	"Paolo Bonzini" <pbonzini@redhat.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>,
	"H. Peter Anvin" <hpa@zytor.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 14:42:18 -0500	[thread overview]
Message-ID: <3dc50409-60dd-ad47-f971-448191e66038__46364.2922732562$1487015003$gmane$org@redhat.com> (raw)
In-Reply-To: <20170213105343.GJ6536@twins.programming.kicks-ass.net>

On 02/13/2017 05:53 AM, Peter Zijlstra 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;
>
> ?

Yes, that looks good to me.

Cheers,
Longman

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