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

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?

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.

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