"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
> On Mon, Oct 08, 2012 at 12:51:25PM +1030, Rusty Russell wrote:
>> Note before anyone gets confused; we were talking about using the PCI
>> config space to indicate what BAR(s) the virtio stuff is in. An
>> alternative would be to simply specify a new layout format in BAR1.
>
> One problem we are still left with is this: device specific
> config accesses are still non atomic.
> This is a problem for multibyte fields such as MAC address
> where MAC could change while we are accessing it.
It's also a problem for related fields, eg. console width and height, or
disk geometry.
> I was thinking about some backwards compatible way to solve this, but if
> we are willing to break compatiblity or use some mode switch, how about
> we give up on virtio config space completely, and do everything besides
> IO and ISR through guest memory?
I think there's still a benefit in the simple publishing of information:
I don't really want to add a control queue for the console. But
inevitably, once-static information can change in later versions, and
it's horrible to have config information plus a bit that says "don't use
this, use the control queue".
Here's a table from a quick audit:
Driver Config Device changes Driver writes... after init?
net Y Y N N
block Y Y Y Y
console Y Y N N
rng N N N N
balloon Y Y Y Y
scsi Y N Y N
9p Y N N N
For config space reads, I suggest the driver publish a generation count.
For writes, the standard seems to be a commit latch. We could abuse the
generation count for this: the driver writes to it to commit config
changes.
ie:
/* Fields in VIRTIO_PCI_CAP_COMMON_CFG: */
struct virtio_pci_common_cfg {
/* About the whole device. */
__le32 device_feature_select; /* read-write */
__le32 device_feature; /* read-only */
__le32 guest_feature_select; /* read-write */
__le32 guest_feature; /* read-only */
__le32 config_gen_and_latch; /* read-write */
__le16 msix_config; /* read-write */
__u8 device_status; /* read-write */
__u8 unused;
/* About a specific virtqueue. */
__le16 queue_select; /* read-write */
__le16 queue_align; /* read-write, power of 2. */
__le16 queue_size; /* read-write, power of 2. */
__le16 queue_msix_vector;/* read-write */
__le64 queue_address; /* read-write: 0xFFFFFFFFFFFFFFFF == DNE. */
};
Thoughts?
Rusty.
PS. Let's make all the virtio-device config LE, too...
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