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:

quoted

    * 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.

quoted

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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