On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > On Mon, Oct 04, 2021 at 05:50:44PM +0200, Cornelia Huck wrote: >> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: >> >> > On Mon, Oct 04, 2021 at 04:33:21PM +0200, Cornelia Huck wrote: >> >> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: >> >> >> >> > On Mon, Oct 04, 2021 at 02:19:55PM +0200, Cornelia Huck wrote: >> >> >> >> >> >> [cc:qemu-devel] >> >> >> >> >> >> On Sat, Oct 02 2021, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: >> >> >> >> >> >> > ok so that's a QEMU bug. Any virtio 1.0 and up >> >> >> > compatible device must use LE. >> >> >> > It can also present a legacy config space where the >> >> >> > endian depends on the guest. >> >> >> >> >> >> So, how is the virtio core supposed to determine this? A >> >> >> transport-specific callback? >> >> > >> >> > I'd say a field in VirtIODevice is easiest. >> >> >> >> The transport needs to set this as soon as it has figured out whether >> >> we're using legacy or not. >> > >> > Basically on each device config access? >> >> Prior to the first one, I think. It should not change again, should it? > > Well yes but we never prohibited someone from poking at both .. > Doing it on each access means we don't have state to migrate. Yes; if it isn't too high overhead, that's probably the safest way to handle it. > >> > >> >> I guess we also need to fence off any >> >> accesses respectively error out the device if the driver tries any >> >> read/write operations that would depend on that knowledge? >> >> >> >> And using a field in VirtIODevice would probably need some care when >> >> migrating. Hm... >> > >> > It's just a shorthand to minimize changes. No need to migrate I think. >> >> If we migrate in from an older QEMU, we don't know whether we are >> dealing with legacy or not, until feature negotiation is already >> done... don't we have to ask the transport? > > Right but the only thing that can happen is config access. Checking on each config space access would be enough then. > Well and for legacy a kick I guess. I think any driver that does something that is not config space access, status access, or feature bit handling without VERSION_1 being set is neccessarily legacy? Does that really need special handling?
