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Re: [Xen-devel] [PATCH 4/6] xen/hypercall: Cope with -ERESTART on more hypercall paths
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 9 Dec 2019 17:43:29 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 09 Dec 2019 17:43:41 +0000
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On 09/12/2019 16:29, Jan Beulich wrote:
> On 09.12.2019 17:25, Jan Beulich wrote:
>> On 05.12.2019 23:30, Andrew Cooper wrote:
>>> These hypercalls each use continue_hypercall_on_cpu(), whose API is about to
>>> switch to use -ERESTART. Update the soon-to-be affected paths to cope,
>>> folding existing contination logic where applicable.
>>>
>>> In addition:
>>> * For platform op and sysctl, insert a cpu_relax() into what is otherwise a
>>> tight spinlock loop, and make the continuation logic common at the
>>> epilogue.
>> Is this really needed with a hypercall_preempt_check() invocation
>> already in the bodies of these loops?
Yes. The reason you're supposed to pause is to stop having memory
traffic constantly trying to pull the spinlock's cacheline into shared
state.
Racing round a tight loop constantly reading 4 or 5 memory locations is
almost as bad.
> And if it's really to be added, shouldn't it be at the bottom
> of the loop bodies rather than at the top?
It doesn't matter.
~Andrew
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