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Re: [PATCH 0/5] xen/wait: Improvements
- To: Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Mon, 18 Jul 2022 11:24:18 +0000
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- Thread-topic: [PATCH 0/5] xen/wait: Improvements
On 18/07/2022 12:11, Jan Beulich wrote:
> On 18.07.2022 09:18, Andrew Cooper wrote:
>> This started out as patch 2 for a different task, and quickly identified some
>> technical debt, long overdue for cleaning up.
>>
>> Andrew Cooper (5):
>> xen/wait: Drop vestigial remnants of TRAP_regs_partial
>> xen/wait: Extend the description of how this logic actually works
>> xen/wait: Minor asm improvements
>> xen/wait: Use relative stack adjustments
>> xen/wait: Remove VCPU_AFFINITY_WAIT
> While going through this series I came to notice that we build wait.c even
> on Arm, and I couldn't convince myself that wait_event() is actually not
> reachable there when e.g. there's an Arm-specific vm_event.c. I would
> generally prefer if non-functioning code paths were actually compiled out.
>
> Thoughts?
I've been wanting to delete wait.c fully for a long time.
It is only needed because the event/paging/access interfaces still use a
single 4k shared ring, and even then, only when you happen to fill the
4k ring, which is 11 vCPUs on x86 last I checked.
I could easily believe that limit has never been tripped on ARM.
There are plenty of perf wins to be had by building a new Xen=>agent
interface using acquire_resource, which include being able to guarantee
that we never need to pause a vCPU to wait for space in the ring to post
a event.
With the interface fixed, wait.c ceases to have any use whatsoever.
~Andrew
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