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bhyve virtio needs barriers

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Under certain tight race conditions, we found that the lack of a memory
barrier in bhyve's virtio handling causes it to miss a NO_NOTIFY state
transition on block devices, resulting in guest stall. The investigation
is recorded in OS-7613. As part of the examination into bhyve's use of
barriers, one other section was found to be problematic, but only on
non-x86 ISAs with less strict memory ordering. That was addressed in
this patch as well, although it was not at all a problem on x86.

PR:		231117
Submitted by:	Patrick Mooney <patrick.mooney@joyent.com>
Reviewed by:	jhb, kib, rgrimes
Approved by:	jhb
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D19501
This commit is contained in:
Rodney W. Grimes 2019-05-18 19:32:38 +00:00
parent 1935b8f92e
commit efc25f65e2
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=347960
1 changed files with 16 additions and 0 deletions
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16
usr.sbin/bhyve/virtio.c
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@ -3,6 +3,7 @@
*
* Copyright (c) 2013 Chris Torek <torek @ torek net>
* All rights reserved.
* Copyright (c) 2019 Joyent, Inc.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
@ -32,6 +33,8 @@ __FBSDID("$FreeBSD$");
#include <sys/param.h>
#include <sys/uio.h>
#include <machine/atomic.h>
#include <stdio.h>
#include <stdint.h>
#include <pthread.h>
@ -422,6 +425,12 @@ vq_relchain(struct vqueue_info *vq, uint16_t idx, uint32_t iolen)
vue = &vuh->vu_ring[uidx++ & mask];
vue->vu_idx = idx;
vue->vu_tlen = iolen;
/*
* Ensure the used descriptor is visible before updating the index.
* This is necessary on ISAs with memory ordering less strict than x86.
*/
atomic_thread_fence_rel();
vuh->vu_idx = uidx;
}
@ -459,6 +468,13 @@ vq_endchains(struct vqueue_info *vq, int used_all_avail)
vs = vq->vq_vs;
old_idx = vq->vq_save_used;
vq->vq_save_used = new_idx = vq->vq_used->vu_idx;
/*
* Use full memory barrier between vu_idx store from preceding
* vq_relchain() call and the loads from VQ_USED_EVENT_IDX() or
* va_flags below.
*/
atomic_thread_fence_seq_cst();
if (used_all_avail &&
(vs->vs_negotiated_caps & VIRTIO_F_NOTIFY_ON_EMPTY))
intr = 1;