Re: [RFC PATCH] vdpa: mandate 1.0 device

From: Jason Wang
Date: Wed Apr 21 2021 - 04:17:29 EST


在 2021/4/21 下午4:03, Michael S. Tsirkin 写道:
On Wed, Apr 21, 2021 at 03:41:36PM +0800, Jason Wang wrote:
在 2021/4/12 下午5:23, Jason Wang 写道:
在 2021/4/12 下午5:09, Michael S. Tsirkin 写道:
On Mon, Apr 12, 2021 at 02:35:07PM +0800, Jason Wang wrote:
在 2021/4/10 上午12:04, Michael S. Tsirkin 写道:
On Fri, Apr 09, 2021 at 12:47:55PM +0800, Jason Wang wrote:
在 2021/4/8 下午11:59, Michael S. Tsirkin 写道:
On Thu, Apr 08, 2021 at 04:26:48PM +0800, Jason Wang wrote:
This patch mandates 1.0 for vDPA devices. The goal is to have the
semantic of normative statement in the virtio
spec and eliminate the
burden of transitional device for both vDPA bus and vDPA parent.

uAPI seems fine since all the vDPA parent mandates
VIRTIO_F_ACCESS_PLATFORM which implies 1.0 devices.

For legacy guests, it can still work since Qemu will mediate when
necessary (e.g doing the endian conversion).

Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
Hmm. If we do this, don't we still have a problem with
legacy drivers which don't ack 1.0?
Yes, but it's not something that is introduced in this
commit. The legacy
driver never work ...
My point is this neither fixes or prevents this.

So my suggestion is to finally add ioctls along the lines
of PROTOCOL_FEATURES of vhost-user.

Then that one can have bits for legacy le, legacy be and modern.

BTW I looked at vhost-user and it does not look like that
has a solution for this problem either, right?
Right.


Note 1.0 affects ring endianness which is not mediated in QEMU
so QEMU can't pretend to device guest is 1.0.
Right, I plan to send patches to do mediation in the
Qemu to unbreak legacy
drivers.

Thanks
I frankly think we'll need PROTOCOL_FEATURES anyway, it's
too useful ...
so why not teach drivers about it and be done with it? You
can't emulate
legacy on modern in a cross endian situation because of vring
endian-ness ...
So the problem still. This can only work when the hardware can support
legacy vring endian-ness.

Consider:

1) the leagcy driver support is non-normative in the spec
2) support a transitional device in the kenrel may requires the
hardware
support and a burden of kernel codes

I'd rather simply drop the legacy driver support
My point is this patch does not drop legacy support. It merely mandates
modern support.

I am not sure I get here. This patch fails the set_feature if VERSION_1
is not negotiated. This means:

1) vDPA presents a modern device instead of transitonal device
2) legacy driver can't be probed

What I'm missing?

Hi Michael:

Do you agree to find the way to present modern device? We need a conclusion
to make the netlink API work to move forward.

Thanks
I think we need a way to support legacy with no data path overhead. qemu
setting VERSION_1 for a legacy guest affects the ring format so it does
not really work. This seems to rule out emulating config space entirely
in userspace.


So I'd rather drop the legacy support in this case. It never work for vDPA in the past and virtio-vDPA doesn't even need that. Note that ACCESS_PLATFORM is mandated for all the vDPA parents right now which implies modern device and LE. I wonder what's the value for supporting legacy in this case or do we really encourage vendors to ship card with legacy support (e.g endian support in the hardware)?



So I think we should add an ioctl along the lines of
protocol features. Then I think we can reserve feature bits
for config space format: legacy LE, legacy BE, modern.


We had VHOST_SET_VRING_ENDIAN but this will complicates both the vDPA parent and management. What's more important, legacy behaviour is not restrictied by the spec.



Querying the feature bits will provide us with info about
what does the device support. Acking them will tell device
what does guest need.


I think this can work, but I wonder how much we can gain from such complexitiy.

Thanks








to have a simple and easy
abstarction in the kenrel. For legacy driver in the guest,
hypervisor is in
charge of the mediation:

1) config space access endian conversion
2) using shadow virtqueue to change the endian in the vring

Thanks
I'd like to avoid shadow virtqueue hacks if at all possible.
Last I checked performance wasn't much better than just emulating
virtio in software.

I think the legacy driver support is just a nice to have. Or do you see
any value to that? I guess for mellanox and intel, only modern device is
supported in the hardware.

Thanks




---
    include/linux/vdpa.h | 6 ++++++
    1 file changed, 6 insertions(+)

diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
index 0fefeb976877..cfde4ec999b4 100644
--- a/include/linux/vdpa.h
+++ b/include/linux/vdpa.h
@@ -6,6 +6,7 @@
    #include <linux/device.h>
    #include <linux/interrupt.h>
    #include <linux/vhost_iotlb.h>
+#include <uapi/linux/virtio_config.h>
    /**
     * vDPA callback definition.
@@ -317,6 +318,11 @@ static inline int
vdpa_set_features(struct vdpa_device *vdev, u64
features)
    {
            const struct vdpa_config_ops *ops = vdev->config;
+        /* Mandating 1.0 to have semantics of
normative statements in
+         * the spec. */
+        if (!(features & BIT_ULL(VIRTIO_F_VERSION_1)))
+        return -EINVAL;
+
        vdev->features_valid = true;
            return ops->set_features(vdev, features);
    }
--
2.25.1