netdev - Re: [virtio-dev] repost: af_packet vs virtio (was packed ring layout proposal v2)

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Date:   Mon, 7 Aug 2017 12:16:23 +0800
From:   Adam Tao <taozhe1@...wei.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        <g@...y3t003839711.china.huawei.com>
CC:     Steven Luong <sluong99@...il.com>, <john.fastabend@...il.com>,
        <alexei.starovoitov@...il.com>, <netdev@...r.kernel.org>,
        <virtio-dev@...ts.oasis-open.org>,
        <virtualization@...ts.linux-foundation.org>, <kvm@...r.kernel.org>
Subject: Re: [virtio-dev] repost: af_packet vs virtio (was packed ring layout
 proposal v2)

On Wed, Aug 02, 2017 at 04:50:03PM +0300, Michael S. Tsirkin wrote:
> On Tue, Aug 01, 2017 at 08:54:27PM -0700, Steven Luong wrote:
> >     * Descriptor ring:
> > 
> >     Guest adds descriptors with unique index values and DESC_HW set in flags.
> >     Host overwrites used descriptors with correct len, index, and DESC_HW
> >     clear.? Flags are always set/cleared last.
> > 
> >     #define DESC_HW 0x0080
> > 
> >     struct desc {
> >     ? ? ? ? __le64 addr;
> >     ? ? ? ? __le32 len;
> >     ? ? ? ? __le16 index;
> >     ? ? ? ? __le16 flags;
> >     };
> > 
> >     When DESC_HW is set, descriptor belongs to device. When it is clear,
> >     it belongs to the driver.
> > 
> >     We can use 1 bit to set direction
> >     /* This marks a buffer as write-only (otherwise read-only). */
> >     #define VRING_DESC_F_WRITE? ? ? 2
> > 
> >     * Scatter/gather support
> > 
> >     We can use 1 bit to chain s/g entries in a request, same as virtio 1.0:
> > 
> >     /* This marks a buffer as continuing via the next field. */
next field seems like a structure field in the software, maybe we need
to change the "next field" to "next desc" to avoid misunderstanding.
> > 
> > 
> > This comment here is confusing to me. In 1.0, virtq_desc has the next field.
> > When the flag VRING_DESC_F_NEXT is set, the next entry to continue is specified
> > in the next field.
> > 
> > Here in 1.1, struct desc does not have the next field, only addr, len, index,
> > and flags. So when VRING_DESC_F_NEXT is set in struct desc's flags field, where
> > is the next entry to continue the current descriptor, the entry immediately
> > following the current entry? ie, if the current entry is at index 10 in the
> > descriptor table and its flags is set for VRING_DESC_F_NEXT, is the entry
> > continuing the current entry in index 11?
> > 
> > Steven
> 
> Exactly, you got it right.
> 
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