netdev - RE: [PATCH v5 2/2] virtio-net: use mtu size as buffer length for big packets

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Date:   Wed, 7 Sep 2022 14:33:02 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Gavin Li <gavinl@...dia.com>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "jesse.brandeburg@...el.com" <jesse.brandeburg@...el.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "sridhar.samudrala@...el.com" <sridhar.samudrala@...el.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "loseweigh@...il.com" <loseweigh@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        Gavi Teitz <gavi@...dia.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Si-Wei Liu <si-wei.liu@...cle.com>
Subject: RE: [PATCH v5 2/2] virtio-net: use mtu size as buffer length for big
 packets


> From: Michael S. Tsirkin <mst@...hat.com>
> Sent: Wednesday, September 7, 2022 10:30 AM

[..]
> > > actually how does this waste space? Is this because your device does
> > > not have INDIRECT?
> > VQ is 256 entries deep.
> > Driver posted total of 256 descriptors.
> > Each descriptor points to a page of 4K.
> > These descriptors are chained as 4K * 16.
> 
> So without indirect then? with indirect each descriptor can point to 16
> entries.
> 
With indirect, can it post 256 * 16 size buffers even though vq depth is 256 entries?
I recall that total number of descriptors with direct/indirect descriptors is limited to vq depth.
Was there some recent clarification occurred in the spec to clarify this?