Re: [Qemu-devel] rfc: vhost user enhancements for vm2vm communication

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

On Tue, Sep 01, 2015 at 05:01:07PM -0700, Nakajima, Jun wrote:
> On Tue, Sep 1, 2015 at 9:28 AM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> > On 2015-09-01 18:02, Michael S. Tsirkin wrote:
> ...
> >> You don't need to be able to map all guest memory if you know
> >> guest won't try to allow device access to all of it.
> >> It's a question of how good is the bus address allocator.
> >
> > But those BARs need to allocate a guest-physical address range as large
> > as the other guest's RAM is, possibly even larger if that RAM is not
> > contiguous, and you can't put other resources into potential holes
> > because VM2 does not know where those holes will be.
> >
> 
> I think you can allocate such guest-physical address ranges
> efficiently if each BAR sets the base of each memory region reported
> by VHOST_SET_MEM_TABLE, for example.  The issue is that we would need
> to 8 (VHOST_MEMORY_MAX_NREGIONS) of them vs. 6 (defined by PCI-SIG).

Besides, 8 is not even a limit: we merged a patch that allows makeing it
larger.

> -- 
> Jun
> Intel Open Source Technology Center