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:

> The following reply was made to PR kern/57199; it has been noted by GNATS.
>
> From: Taylor R Campbell <riastradh%NetBSD.org@localhost>
> To: Brad Spencer <brad%anduin.eldar.org@localhost>,
> 	Manuel Bouyer <bouyer%antioche.eu.org@localhost>
> Cc: gnats-bugs%NetBSD.org@localhost, netbsd-bugs%NetBSD.org@localhost, gdt%lexort.com@localhost
> Subject: Re: port-xen/57199: Pure PVH i386 guests hang on disk activity
> Date: Mon, 15 Jul 2024 17:33:17 +0000
>
>  I can think of two ways this patch could have an impact:
>  
>  1. Some Xen driver relies on write-combining memory (i.e.,
>     `prefetchable' in PCIese and bus_dmaese), or on non-temporal
>     stores.  This seems unlikely.
>  
>  2. This is a single-(v)CPU system which has patched out the lock
>     prefix in membar_sync.
>  
>  Unless (1) is happening, I doubt there's any reason to need mfence,
>  lfence, or sfence -- except in the circumstances of (1), mfence is
>  just a more expensive version of a locked-add for store-before-load
>  ordering, and lfence and sfence are never necessary.  See, e.g., the
>  AMD memory access ordering rules table:
>  
>  AMD64 Architecture Programmer's Manual, Volume 2: System Programming,
>  24593--Rev. 3.38--November 2021, Sec. 7.4.2 Memory Barrier Interaction
>  with Memory Types, Table 7-3, p. 196.
>  https://web.archive.org/web/20220625040004/https://www.amd.com/system/files=
>  /TechDocs/24593.pdf#page=3D256
>  
>  
>  Is this a single-(v)CPU system?  Can you enter crash(8) or drop into
>  ddb and disassemble the membar_sync function?  I bet you'll find no
>  lock prefix there, which would explain the hangs.

Yes, it is a single vcpu DOMU.  I can get into ddb without any problems
but would need the commands to do the disassembly.

Context reminder.. this is a 32 bit i386 DOMU running PVH mode that
hangs up.  For reasons that are not clear to me, a i386 guest running
PV+PVSHIM does not hang.  A 64 bit PVH DOMU does not hang.

>  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.
>  


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