Linux Virtualization - Aug 2017 - [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.