linux-kernel - Re: vhost changes (batched) in linux-next after 12/13 trigger random crashes in KVM guests after reboot

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Date:   Tue, 11 Feb 2020 14:04:54 +0100
From:   Eugenio Pérez <eperezma@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>
Subject: Re: vhost changes (batched) in linux-next after 12/13 trigger
 random crashes in KVM guests after reboot

On Mon, 2020-02-10 at 12:01 +0100, Christian Borntraeger wrote:
> 
> On 10.02.20 10:47, Eugenio Perez Martin wrote:
> > Hi Christian.
> > 
> > I'm not able to reproduce the failure with eccb852f1fe6bede630e2e4f1a121a81e34354ab commit. Could you add more data?
> > Your configuration (libvirt or qemu line), and host's dmesg output if any?
> > 
> > Thanks!
> 
> If it was not obvious, this is on s390x, a big endian system.
> 

Hi Christian. Thank you very much for your fast responses.

Could you try this patch on top of eccb852f1fe6bede630e2e4f1a121a81e34354ab?

Thanks!

>From 71d0f9108a18aa894cc0c0c1c7efbad39f465a27 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <
eperezma@...hat.com>
Date: Tue, 11 Feb 2020 13:19:10 +0100
Subject: [PATCH] vhost: fix return value of vhost_get_vq_desc

Before of the batch change, it was the chain's head. Need to keep that
way or we will not be able to free a chain of descriptors.

Fixes: eccb852f1fe6 ("vhost: batching fetches")
---
 drivers/vhost/vhost.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index b5a51b1f2e79..fc422c3e5c08 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2409,12 +2409,11 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 			*out_num += ret;
 		}
 
-		ret = desc->id;
-
 		if (!(desc->flags & VRING_DESC_F_NEXT))
 			break;
 	}
 
+	ret = vq->descs[vq->first_desc].id;
 	vq->first_desc = i + 1;
 
 	return ret;
-- 
2.18.1