virtio — archive

[Michael S. Tsirkin <mst@redhat.com>]

On Sun, Apr 12, 2015 at 03:38:07PM +0000, James Bottomley wrote: > On Sun, 2015-04-12 at 17:28 +0200, Michael S. Tsirkin wrote: > > On Tue, Apr 07, 2015 at 10:40:50PM +0000, James Bottomley wrote: > > > On Tue, 2015-04-07 at 12:02 +0200, Michael S. Tsirkin wrote: > > > > On Tue, Apr 07, 2015 at 11:19:08AM +0200, Michael S. Tsirkin wrote: > > > > > On Mon, Apr 06, 2015 at 06:17:56PM +0000, James Bottomley wrote: > > > > > > On Thu, 2015-04-02 at 12:23 +0200, Cornelia Huck wrote: > > > > > > > On Thu, 2 Apr 2015 12:06:41 +0200 > > > > > > > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > > > > > > > > > > > > > On Thu, Apr 02, 2015 at 11:01:45AM +0200, Cornelia Huck wrote: > > > > > > > > > On Wed, 1 Apr 2015 20:42:13 +0200 > > > > > > > > > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > > > > > > > > > > > > > > > +For device configuration access, the driver MUST use 8-bit wide > > > > > > > > > > +accesses for 8-bit wide fields, 16-bit wide and aligned accesses > > > > > > > > > > +for 16-bit wide fields and 32-bit wide and aligned accesses for > > > > > > > > > > +32-bit and 64-bit wide fields. For 64-bit fields, the driver MAY > > > > > > > > > > +access each of the high and low 32-bit parts of the field > > > > > > > > > > +independently. > > > > > > > > > > > > > > > > > > Doesn't the last sentence follow from "use 32-bit wide accesses" > > > > > > > > > already? > > > > > > > > > > > > > > > > Someone might assume that driver must always access low part, > > > > > > > > then high part. Last sentence says it does not have to. > > > > > > > > > > > > > > > > > > > > > > OK, that clears it up then. > > > > > > > > > > > > Not entirely. There's a hidden gotcha here in terms of fields which > > > > > > perform actions. For instance the status register is used to transmit > > > > > > actions and information from the guest to the host. Problems happen if > > > > > > any action register is wider than 32 bits, because a 32 bit OS cannot > > > > > > now set it atomically. > > > > > > > > > > > > There a various ways around this: we could say no action register may > > > > > > ever be wider than 32 bits (easy) or we could prescribe a mechanism for > > > > > > using 64 bit action registers on 32 bits (locking + defined initiation, > > > > > > say writing to the lower 32 bits triggers the action, so on 32 bit > > > > > > architectures, you lock, write upper, then write lower, then unlock). > > > > > > > > > > > > I'd actually say it's useful clarification to code the former into the > > > > > > Spec until such time as someone finds a need for a 64 bit action > > > > > > register. > > > > > > > > > > > > James > > > > > > > > > > > > > > > > This seems to belong to appendix B: creating new device types. > > > > > We already have this text there: > > > > > Remember that configuration fields over 32 bits wide might not be > > > > > atomically writable by the driver. > > > > > > > > > > Is this sufficient? If not, we can add > > > > > Therefore, it is generally not a good idea to have > > > > > such writeable fields, and especially to have > > > > > driver writes into such fields trigger device actions". > > > > > > > > > > Is this what you had in mind? > > > > > > > > If yes I'd prefer that you change your vote to yes, > > > > with a comment, and open another issue to add a note. > > > > > > Well, I'd prefer the updates went in atomically, but I think this will > > > work as a compromise. > > > > > > James > > > > > > > Created VIRTIO-140 for this. > > Will post a patch when I can post to the list (still having trouble with > our email rebranding). > > James > Since you are a member of the TC, I don't think there are any IPR issues if you just send any email to me, I'll forward. OTOH if you just want that text added, I can send that patch myself.