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Re: [PATCH] xen/wait: Describe RSB safety
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Fri, 5 Aug 2022 11:10:17 +0000
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- Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 05 Aug 2022 11:10:52 +0000
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- Thread-topic: [PATCH] xen/wait: Describe RSB safety
On 05/08/2022 11:51, Jan Beulich wrote:
> On 05.08.2022 12:38, Andrew Cooper wrote:
>> It turns out that we do in fact have RSB safety here, but not for obvious
>> reasons.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> preferably with ...
>
>> --- a/xen/common/wait.c
>> +++ b/xen/common/wait.c
>> @@ -210,6 +210,26 @@ void check_wakeup_from_wait(void)
>> }
>>
>> /*
>> + * We are about to jump into a deeper call tree. In principle, this
>> risks
>> + * executing more RET than CALL instructions, and underflowing the RSB.
>> + *
>> + * However, we are pinned to the same CPU as previously. Therefore,
>> + * either:
>> + *
>> + * 1) We've scheduled another vCPU in the meantime, and the context
>> + * switch path has (by default) issued IPBP which flushes the RSB,
>> or
> ... IBPB used here and ...
>
>> + * 2) We're still in the same context. Returning back to the deeper
>> + * call tree is resuming the execution path we left, and remains
>> + * balanced as far as that logic is concerned.
>> + *
>> + * In fact, the path though the scheduler will execute more CALL
>> than
> ... (nit) "through" used here.
Wow I failed at writing... Fixed.
~Andrew
>
>> + * RET instructions, making the RSB unbalanced in the safe
>> direction.
>> + *
>> + * Therefore, no actions are necessary here to maintain RSB safety.
>> + */
>> +
>> + /*
>> * Hand-rolled longjmp().
>> *
>> * check_wakeup_from_wait() is always called with a shallow stack,
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