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

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

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.
 
 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.