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

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Message-ID: <20170802164931-mutt-send-email-mst@kernel.org>
Date:   Wed, 2 Aug 2017 16:50:03 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Steven Luong <sluong99@...il.com>
Cc:     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 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. */
> 
> 
> 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.