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Re: [PATCH 2/5] xen/wait: Extend the description of how this logic actually works
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Mon, 18 Jul 2022 10:11:16 +0000
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- Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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- Thread-topic: [PATCH 2/5] xen/wait: Extend the description of how this logic actually works
On 18/07/2022 11:00, Jan Beulich wrote:
> On 18.07.2022 09:18, Andrew Cooper wrote:
>> @@ -199,9 +211,18 @@ void check_wakeup_from_wait(void)
>> }
>>
>> /*
>> - * Hand-rolled longjmp(). Returns to __prepare_to_wait(), and lands on
>> a
>> - * `rep movs` instruction. All other GPRs are restored from the stack,
>> so
>> - * are available for use here.
>> + * Hand-rolled longjmp().
>> + *
>> + * check_wakeup_from_wait() is always called with a shallow stack,
>> + * immediately after the vCPU has been rescheduled.
>> + *
>> + * Adjust %rsp to be the correct depth for the (deeper) stack we want to
>> + * restore, then prepare %rsi, %rdi and %rcx such that when we intercept
>> + * the rep movs in __prepare_to_wait(), it copies from wqv->stack over
>> the
>> + * active stack.
> I'm struggling with the use of "intercept" here, but I guess that's just
> because I'm not a native speaker.
"intercept" is the same terminology used in the middle of
__prepare_to_wait()'s block.
It's because we have a weird setup where this is (now) a noreturn
function merging into the middle of a function which already executed once.
I'm happy to change it if it's unclear, but I can't think of a better
description.
>> + * All other GPRs are available for use; they're either restored from
>> + * wqv->stack or explicitly clobbered.
> You talking of "other GPRs" - there aren't any which are explicitly
> clobbered. It's only the previously named ones which are. Hence I'd like
> to ask that the respective parts of the sentence be dropped. Then
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
It becomes true in the next patch. I'll try and shuffle things.
~Andrew
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