From mboxrd@z Thu Jan  1 00:00:00 1970
From: Waiman Long <longman@redhat.com>
Subject: Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a
	callee-save function
Date: Mon, 13 Feb 2017 17:34:01 -0500
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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, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>, Paolo Bonzini <pbonzini@redhat.com>, 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>
List-Id: virtualization@lists.linuxfoundation.org

On 02/13/2017 04:52 PM, Peter Zijlstra wrote:
> On Mon, Feb 13, 2017 at 03:12:45PM -0500, Waiman Long wrote:
>> On 02/13/2017 02:42 PM, Waiman Long wrote:
>>> 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
>>>
>> Sorry, I am going to take it back. The displacement or offset can only
>> be up to 32-bit. So we will still need to use at least one more
>> register, I think.
> I don't think that would be a problem, I very much doubt we declare more
> than 4G worth of per-cpu variables in the kernel.
>
> In any case, use "e" or "Z" as constraint (I never quite know when to
> use which). That are s32 and u32 displacement immediates resp. and
> should fail compile with a semi-sensible failure if the displacement is
> too big.
>
It is the address of &steal_time that will exceed the 32-bit limit.

Cheers,
Longman