From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH 0/2] make hypercall preemption checks
	consistent
Date: Tue, 4 Mar 2014 12:10:56 +0000
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To: Jan Beulich <JBeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>, Keir Fraser <keir@xen.org>, xen-devel <xen-devel@lists.xenproject.org>
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On 04/03/14 12:00, Jan Beulich wrote:
>>>> On 04.03.14 at 12:52, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> On 04/03/14 11:21, Jan Beulich wrote:
>>> - never preempt on the first iteration (ensure forward progress)
>>> - never preempt on the last iteration (pointless/wasteful)
>>> - do cheap checks first
>>>
>>> 1: common: make hypercall preemption checks consistent
>>> 2: x86: make hypercall preemption checks consistent
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>
>> All in all, this is a good improvement over what is currently present.
>>
>> However, given the overhead of creating continuations (particularly for
>> 32bit HVM guests, which have been seen to unconditionally fail the
>> preemption check by the time the compat layer has run), some of these
>> operations would probably be better having more than a single guaranteed
>> operation.
> 
> I agree, but I wanted to do one step at a time. Judging how much
> work we want to permit done between preemption points will be
> either heavy guess work, or require quite a bit of performance
> measurement...

Perhaps something time-based?  Record the time at start and make
hypercall_preempt_check() return true if more than T time has elapsed?

David