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Date:   Mon, 5 Jun 2023 14:54:20 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>,
        Eugenio =?utf-8?B?UMOpcmV6?= <eperezma@redhat.com>,
        Tiwei Bie <tiwei.bie@intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature
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References: <20230605110644.151211-1-sgarzare@redhat.com>
 <20230605084104-mutt-send-email-mst@kernel.org>
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On Mon, Jun 05, 2023 at 08:41:54AM -0400, Michael S. Tsirkin wrote:
>On Mon, Jun 05, 2023 at 01:06:44PM +0200, Stefano Garzarella wrote:
>> vhost-vdpa IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_BASE)
>> don't support packed virtqueue well yet, so let's filter the
>> VIRTIO_F_RING_PACKED feature for now in vhost_vdpa_get_features().
>>
>> This way, even if the device supports it, we don't risk it being
>> negotiated, then the VMM is unable to set the vring state properly.
>>
>> Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>>
>> Notes:
>>     This patch should be applied before the "[PATCH v2 0/3] vhost_vdpa:
>>     better PACKED support" series [1] and backported in stable branches.
>>
>>     We can revert it when we are sure that everything is working with
>>     packed virtqueues.
>>
>>     Thanks,
>>     Stefano
>>
>>     [1] https://lore.kernel.org/virtualization/20230424225031.18947-1-shannon.nelson@amd.com/
>
>I'm a bit lost here. So why am I merging "better PACKED support" then?

To really support packed virtqueue with vhost-vdpa, at that point we 
would also have to revert this patch.

I wasn't sure if you wanted to queue the series for this merge window.
In that case do you think it is better to send this patch only for 
stable branches?

>Does this patch make them a NOP?

Yep, after applying the "better PACKED support" series and being sure 
that the IOCTLs of vhost-vdpa support packed virtqueue, we should revert 
this patch.

Let me know if you prefer a different approach.

I'm concerned that QEMU uses vhost-vdpa IOCTLs thinking that the kernel 
interprets them the right way, when it does not.

Thanks,
Stefano

>
>>  drivers/vhost/vdpa.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
>> index 8c1aefc865f0..ac2152135b23 100644
>> --- a/drivers/vhost/vdpa.c
>> +++ b/drivers/vhost/vdpa.c
>> @@ -397,6 +397,12 @@ static long vhost_vdpa_get_features(struct vhost_vdpa *v, u64 __user *featurep)
>>
>>  	features = ops->get_device_features(vdpa);
>>
>> +	/*
>> +	 * IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_BASE) don't support
>> +	 * packed virtqueue well yet, so let's filter the feature for now.
>> +	 */
>> +	features &= ~BIT_ULL(VIRTIO_F_RING_PACKED);
>> +
>>  	if (copy_to_user(featurep, &features, sizeof(features)))
>>  		return -EFAULT;
>>
>>
>> base-commit: 9561de3a55bed6bdd44a12820ba81ec416e705a7
>> --
>> 2.40.1
>