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Re: [Xen-devel] [PATCH 0/2] make hypercall preemption checks consistent

>>> On 04.03.14 at 13:10, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> On 04/03/14 12:00, Jan Beulich wrote:
>>>>> On 04.03.14 at 12:52, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxxx>
>>> 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?

That's certainly an interesting idea. But it doesn't remove the need
to determine the actual value of the parameter to use (T in this case).


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