vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero)

Sorry for reviving this ancient thread. I was kinda lost for the 
conclusion it ended up with. I have the following questions,

1. legacy guest support: from the past conversations it doesn't seem the 
support will be completely dropped from the table, is my understanding 
correct? Actually we're interested in supporting virtio v0.95 guest for 
x86, which is backed by the spec at 
https://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf. Though I'm not 
sure if there's request/need to support wilder legacy virtio versions 
earlier beyond.

2. suppose some form of legacy guest support needs to be there, how do 
we deal with the bogus assumption below in vdpa_get_config() in the 
short term? It looks one of the intuitive fix is to move the 
vdpa_set_features call out of vdpa_get_config() to vdpa_set_config().

         /*
          * Config accesses aren't supposed to trigger before features 
are set.
          * If it does happen we assume a legacy guest.
          */
         if (!vdev->features_valid)
                 vdpa_set_features(vdev, 0);
         ops->get_config(vdev, offset, buf, len);

I can post a patch to fix 2) if there's consensus already reached.

Thanks,
-Siwei

On 3/2/2021 2:53 AM, Jason Wang wrote:

On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:

quoted

On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:

quoted

On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:

quoted

On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:

quoted

quoted

Detecting it isn't enough though, we will need a new ioctl to notify
the kernel that it's a legacy guest. Ugh :(

Well, although I think adding an ioctl is doable, may I know what 
the use
case there will be for kernel to leverage such info directly? Is 
there a
case QEMU can't do with dedicate ioctls later if there's indeed
differentiation (legacy v.s. modern) needed?

BTW a good API could be

#define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
#define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)

we did it per vring but maybe that was a mistake ...

Actually, I wonder whether it's good time to just not support legacy 
driver
for vDPA. Consider:

1) It's definition is no-normative
2) A lot of budren of codes

So qemu can still present the legacy device since the config space 
or other
stuffs that is presented by vhost-vDPA is not expected to be 
accessed by
guest directly. Qemu can do the endian conversion when necessary in 
this
case?

Thanks

Overall I would be fine with this approach but we need to avoid breaking
working userspace, qemu releases with vdpa support are out there and
seem to work for people. Any changes need to take that into account
and document compatibility concerns.

Agree, let me check.

quoted

  I note that any hardware
implementation is already broken for legacy except on platforms with
strong ordering which might be helpful in reducing the scope.

Yes.

Thanks

quoted

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization