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From:   Alvaro Karsz <alvaro.karsz@solid-run.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
CC:     Jason Wang <jasowang@redhat.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "edumazet@google.com" <edumazet@google.com>,
        "kuba@kernel.org" <kuba@kernel.org>,
        "pabeni@redhat.com" <pabeni@redhat.com>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net] virtio-net: reject small vring sizes
Thread-Topic: [PATCH net] virtio-net: reject small vring sizes
Thread-Index: AQHZcDeGnH5xR2OGlkuo4s6jvhNMC68uIjT9gABGFgCAAG90AIAAMSwAgAABjzCAAARBAIAABATIgAAENICAAAM4M4AAIQOAgAALfyGAABuqgIAAAQKHgAADxACACRVQMYAACvaAgAAGduU=
Date:   Sun, 23 Apr 2023 07:52:10 +0000
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> Hmm. I was wrong. There is no way to disable CVQ feature bit.=0A=
> =0A=
> 1. Reset the device.=0A=
> 2. Set the ACKNOWLEDGE status bit: the guest OS has notice the device.=0A=
> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.=
=0A=
> 4. Read device feature bits, and write the subset of feature bits underst=
ood by the OS and driver to the=0A=
> device. During this step the driver MAY read (but MUST NOT write) the dev=
ice-specific configuration=0A=
> fields to check that it can support the device before accepting it.=0A=
> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature=
 bits after this step.=0A=
> 6. Re-read device status to ensure the FEATURES_OK bit is still set: othe=
rwise, the device does not=0A=
> support our subset of features and the device is unusable.=0A=
> 7. Perform device-specific setup, including discovery of virtqueues for t=
he device, optional per-> bus setup,=0A=
> reading and possibly writing the device=92s virtio configuration space, a=
nd population of virtqueues.=0A=
> 8. Set the DRIVER_OK status bit. At this point the device is =93live=94.=
=0A=
> =0A=
> =0A=
> So features are confirmed before find vqs.=0A=
> =0A=
> The rest of stuff can probably just be moved to after find_vqs without=0A=
> much pain.=0A=
> =0A=
Actually, I think that with a little bit of pain :)=0A=
If we use small vrings and a GRO feature bit is set, Linux will need to all=
ocate 64KB of continuous memory for every receive descriptor..=0A=
=0A=
Instead of failing probe if GRO/CVQ are set, can we just reset the device i=
f we discover small vrings and start over?=0A=
Can we remember that this device uses small vrings, and then just avoid neg=
otiating the features that we cannot support?=