netdev - Re: [PATCH net] virtio-net: reject small vring sizes

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Date:   Sun, 23 Apr 2023 07:09:33 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alvaro Karsz <alvaro.karsz@...id-run.com>
Cc:     "jasowang@...hat.com" <jasowang@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] virtio-net: reject small vring sizes

On Mon, Apr 17, 2023 at 06:43:39AM +0000, Alvaro Karsz wrote:
> > > +static int virtnet_validate_vqs(struct virtnet_info *vi)
> > > +{
> > > +     u32 i, min_size = roundup_pow_of_two(MAX_SKB_FRAGS + 2);
> > 
> > why power of two?
> 
> The ring size is always a power of 2,

Not really, packed rings allow non power of 2.
Linux had a bug that it required power of 2 for packed, but
we are fixing that finally.

> so checking against
> MAX_SKB_FRAGS + 2 or against roundup_pow_of_two will result in the
> same, and I think that printing the warning with the actual min value
> is more helpful.  I can check the condition against MAX_SKB_FRAGS + 2,
> and print the rounded value in case of an error.
>