Re: [virtio-comment] [RFC] ivshmem v2: Shared memory device specification

[Jan Kiszka]

On 23.07.20 08:54, Stefan Hajnoczi wrote:
> On Fri, Jul 17, 2020 at 06:15:58PM +0200, Jan Kiszka wrote:
> > On 15.07.20 15:27, Stefan Hajnoczi wrote:
> > > On Mon, May 25, 2020 at 09:58:28AM +0200, Jan Kiszka wrote:
>
> Thanks for the responses. It would be great to update the spec with
> these clarifications.
>
> > > > If BAR 2 is not present, the shared memory region is not relocatable
> > > > by the user. In that case, the hypervisor has to implement the Base
> > > > Address register in the vendor-specific capability.
> > >
> > > What does relocatable mean in this context?
> >
> > That the guest can decide (via BAR) where the resource should show up in the
> > physical guest address space. We do not want to support this in setups like
> > for static partitioning hypervisors, and then we use that side-channel
> > read-only configuration.
>
> I see. I'm not sure what is vendor-specific about non-relocatable shared
> memory. I guess it could be added to the spec too?

That "vendor-specific" comes from the PCI spec which - to my 
understanding - provides us no other means to introduce registers to the 
config space that are outside of the PCI spec. I could introduce a name 
for the ivshmem vendor cap and use that name here - would that be better?

> In any case, since "relocatable" hasn't been fully defined, I suggest
> making the statement more general:
>
>    If BAR 2 is not present the hypervisor has to implement the Base
>    Address Register in the vendor-specific capability. This can be used
>    for vendor-specific shared memory functionality.

Will integrate this.

Thanks,
Jan

--
Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux