Re: port-xen/57199: Pure PVH i386 guests hang on disk activity

[Brad Spencer <brad%anduin.eldar.org@localhost>]

Taylor R Campbell <riastradh%NetBSD.org@localhost> writes:

> [1:text/plain Hide]
>
>> Date: Mon, 15 Jul 2024 17:33:17 +0000
>> From: Taylor R Campbell <riastradh%NetBSD.org@localhost>
>> 
>> 2. This is a single-(v)CPU system which has patched out the lock
>>    prefix in membar_sync.
>> [...]
>> If my hypothesis about (2) is correct, the right thing is probably
>> either to make xen_mb be an assembly stub that does
>> 
>> 	lock
>> 	addq $0,-8(%rsp)
>> 
>> (without the membar_sync hotpatching), or to make xen_mb be inline asm
>> to do the same.
>
> The attached patch implements this approach, and leaves extensive
> comments explaining what's going on, without issuing any unnecessary
> mfence/lfence/sfence instructions.  Can you try it out?
>
> Under my hypothesis, the domU kernel certainly needs this change.  And
> the dom0 kernel may also need it, because I believe it uses
> RING_PUSH_RESPONSES_AND_CHECK_NOTIFY and RING_FINAL_CHECK_FOR_REQUESTS
> which rely on xen_mb too.


a) Manuel's patch allowed the 32-bit PVH DOMU to do the untar without
hanging.

b) I will test Taylor's patch on the DOMU.  Doing that on the DOM0 is
probably not something I can manage right now.


-- 
Brad Spencer - brad%anduin.eldar.org@localhost - KC8VKS - http://anduin.eldar.org