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From: Halil Pasic <>
To: "Michael S. Tsirkin" <>,
	Jason Wang <>,
	Xie Yongji <>,,
Cc: Halil Pasic <>,,,
	Cornelia Huck <>,
	Christian Borntraeger <>,,,
	Raphael Norwitz <>,
Subject: [PATCH v3 1/1] virtio: write back F_VERSION_1 before validate
Date: Mon, 11 Oct 2021 07:39:21 +0200	[thread overview]
Message-ID: <> (raw)

The virtio specification virtio-v1.1-cs01 states: "Transitional devices
MUST detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not
been acknowledged by the driver."  This is exactly what QEMU as of 6.1
has done relying solely on VIRTIO_F_VERSION_1 for detecting that.

However, the specification also says: "... the driver MAY read (but MUST
NOT write) the device-specific configuration fields to check that it can
support the device ..." before setting FEATURES_OK.

In that case, any transitional device relying solely on
VIRTIO_F_VERSION_1 for detecting legacy drivers will return data in
legacy format.  In particular, this implies that it is in big endian
format for big endian guests. This naturally confuses the driver which
expects little endian in the modern mode.

It is probably a good idea to amend the spec to clarify that
VIRTIO_F_VERSION_1 can only be relied on after the feature negotiation
is complete. Before validate callback existed, config space was only
read after FEATURES_OK. However, we already have two regressions, so
let's address this here as well.

The regressions affect the VIRTIO_NET_F_MTU feature of virtio-net and
the VIRTIO_BLK_F_BLK_SIZE feature of virtio-blk for BE guests when
virtio 1.0 is used on both sides. The latter renders virtio-blk unusable
with DASD backing, because things simply don't work with the default.
See Fixes tags for relevant commits.

For QEMU, we can work around the issue by writing out the feature bits
with VIRTIO_F_VERSION_1 bit set.  We (ab)use the finalize_features
config op for this. This isn't enough to address all vhost devices since
these do not get the features until FEATURES_OK, however it looks like
the affected devices actually never handled the endianness for legacy
mode correctly, so at least that's not a regression.

No devices except virtio net and virtio blk seem to be affected.

Long term the right thing to do is to fix the hypervisors.

Cc: <> #v4.11
Signed-off-by: Halil Pasic <>
Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in config space")
Fixes: fe36cbe0671e ("virtio_net: clear MTU when out of range")
Reviewed-by: Cornelia Huck <>

@Connie: I made some more commit message changes to accommodate Michael's
requests. I just assumed these will work or you as well and kept your
r-b. Please shout at me if it needs to be dropped :)
 drivers/virtio/virtio.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 0a5b54034d4b..236081afe9a2 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -239,6 +239,17 @@ static int virtio_dev_probe(struct device *_d)
 		driver_features_legacy = driver_features;
+	/*
+	 * Some devices detect legacy solely via F_VERSION_1. Write
+	 * F_VERSION_1 to force LE config space accesses before FEATURES_OK for
+	 * these when needed.
+	 */
+	if (drv->validate && !virtio_legacy_is_little_endian()
+			  && device_features & BIT_ULL(VIRTIO_F_VERSION_1)) {
+		dev->features = BIT_ULL(VIRTIO_F_VERSION_1);
+		dev->config->finalize_features(dev);
+	}
 	if (device_features & (1ULL << VIRTIO_F_VERSION_1))
 		dev->features = driver_features & device_features;

base-commit: 60a9483534ed0d99090a2ee1d4bb0b8179195f51

             reply	other threads:[~2021-10-11  5:39 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-11  5:39 Halil Pasic [this message]
2021-10-11  9:07 ` Cornelia Huck
2021-10-13 10:10 ` Michael S. Tsirkin
2021-10-13 11:23   ` Christian Borntraeger
2021-10-13 12:24     ` Michael S. Tsirkin
2021-10-13 12:44       ` Halil Pasic
2021-10-13 12:46         ` Michael S. Tsirkin
2021-10-13 12:52       ` Cornelia Huck
2021-10-13 12:55         ` Michael S. Tsirkin

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