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" <> 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" <> 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.
