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Subject: Re: [Xen-devel] [PATCH v2] x86/paravirt: Don't make
 vcpu_is_preempted() a callee-save function
To: Waiman Long <longman@redhat.com>,
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From: Andrew Cooper <andrew.cooper3@citrix.com>
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Date: Tue, 14 Feb 2017 16:18:55 +0000
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On 14/02/17 14:46, Waiman Long wrote:
> On 02/14/2017 04:39 AM, Peter Zijlstra wrote:
>> On Mon, Feb 13, 2017 at 05:34:01PM -0500, Waiman Long wrote:
>>> It is the address of &steal_time that will exceed the 32-bit limit.
>> That seems extremely unlikely. That would mean we have more than 4G
>> worth of per-cpu variables declared in the kernel.
> I have some doubt about if the compiler is able to properly use
> RIP-relative addressing for this. Anyway, it seems like constraints
> aren't allowed for asm() when not in the function context, at least for
> the the compiler that I am using (4.8.5). So it is a moot point.

You can work the issue of not having parameters in a plain asm()
statement by using an asm-offset, stringizing it, and have C put the
string fragments back together.

"cmpb $0, " STR(STEAL_TIME_preempted) "(%rax);"

~Andrew