Hi Michael,
I just noticed this patch got pulled to linux-next prematurely without
getting consensus on code review, am not sure why. Hope it was just an
oversight.
Unfortunately this introduced functionality regression to at least two
cases so far as I see:
1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently exposed
and displayed in "vdpa dev config show" before feature negotiation is
done. Noted the corresponding features name shown in vdpa tool is called
"negotiated_features" rather than "driver_features". I see in no way the
intended change of the patch should break this user level expectation
regardless of any spec requirement. Do you agree on this point?
2. There was also another implicit assumption that is broken by this
patch. There could be a vdpa tool query of config via
vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races with
the first vdpa_set_features() call from VMM e.g. QEMU. Since the
S_FEATURES_OK blocking condition is removed, if the vdpa tool query
occurs earlier than the first set_driver_features() call from VMM, the
following code will treat the guest as legacy and then trigger an
erroneous vdpa_set_features_unlocked(... , 0) call to the vdpa driver:
374 /*
375 * Config accesses aren't supposed to trigger before
features are set.
376 * If it does happen we assume a legacy guest.
377 */
378 if (!vdev->features_valid)
379 vdpa_set_features_unlocked(vdev, 0);
380 ops->get_config(vdev, offset, buf, len);
Depending on vendor driver's implementation, L380 may either return
invalid config data (or invalid endianness if on BE) or only config
fields that are valid in legacy layout. What's more severe is that, vdpa
tool query in theory shouldn't affect feature negotiation at all by
making confusing calls to the device, but now it is possible with the
patch. Fixing this would require more delicate work on the other paths
involving the cf_lock reader/write semaphore.
Not sure what you plan to do next, post the fixes for both issues and
get the community review? Or simply revert the patch in question? Let us
know.
Thanks,
-Siwei
On 8/12/2022 3:44 AM, Zhu Lingshan wrote:
Users may want to query the config space of a vDPA device,
to choose a appropriate one for a certain guest. This means the
users need to read the config space before FEATURES_OK, and
the existence of config space contents does not depend on
FEATURES_OK.
The spec says:
The device MUST allow reading of any device-specific configuration
field before FEATURES_OK is set by the driver. This includes
fields which are conditional on feature bits, as long as those
feature bits are offered by the device.
Signed-off-by: Zhu Lingshan <lingshan.zhu@xxxxxxxxx>
---
drivers/vdpa/vdpa.c | 8 --------
1 file changed, 8 deletions(-)
diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
index 6eb3d972d802..bf312d9c59ab 100644
--- a/drivers/vdpa/vdpa.c
+++ b/drivers/vdpa/vdpa.c
@@ -855,17 +855,9 @@ vdpa_dev_config_fill(struct vdpa_device *vdev, struct sk_buff *msg, u32 portid,
{
u32 device_id;
void *hdr;
- u8 status;
int err;
down_read(&vdev->cf_lock);
- status = vdev->config->get_status(vdev);
- if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
- NL_SET_ERR_MSG_MOD(extack, "Features negotiation not completed");
- err = -EAGAIN;
- goto out;
- }
-
hdr = genlmsg_put(msg, portid, seq, &vdpa_nl_family, flags,
VDPA_CMD_DEV_CONFIG_GET);
if (!hdr) {
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