virtio — archive

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

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. > -- > MST