Re: [PATCH] kvm tools: adds a PCI device that exports a host shared segment as a PCI BAR in the guest

[Pekka Enberg <penberg@kernel.org>]

On Wed, 2011-08-24 at 21:49 -0700, David Evensky wrote:
> On Wed, Aug 24, 2011 at 10:27:18PM -0500, Alexander Graf wrote:
> > 
> > On 24.08.2011, at 17:25, David Evensky wrote:
> > 
> > > 
> > > 
> > > This patch adds a PCI device that provides PCI device memory to the
> > > guest. This memory in the guest exists as a shared memory segment in
> > > the host. This is similar memory sharing capability of Nahanni
> > > (ivshmem) available in QEMU. In this case, the shared memory segment
> > > is exposed as a PCI BAR only.
> > > 
> > > A new command line argument is added as:
> > >    --shmem pci:0xc8000000:16MB:handle=/newmem:create
> > > 
> > > which will set the PCI BAR at 0xc8000000, the shared memory segment
> > > and the region pointed to by the BAR will be 16MB. On the host side
> > > the shm_open handle will be '/newmem', and the kvm tool will create
> > > the shared segment, set its size, and initialize it. If the size,
> > > handle, or create flag are absent, they will default to 16MB,
> > > handle=/kvm_shmem, and create will be false. The address family,
> > > 'pci:' is also optional as it is the only address family currently
> > > supported. Only a single --shmem is supported at this time.
> > 
> > Did you have a look at ivshmem? It does that today, but also gives
> you an IRQ line so the guests can poke each other. For something as
> simple as this, I don't see why we'd need two competing
> implementations.
> 
> Isn't ivshmem in QEMU? If so, then I don't think there isn't any
> competition. How do you feel that these are competing?

It's obviously not competing. One thing you might want to consider is
making the guest interface compatible with ivshmem. Is there any reason
we shouldn't do that? I don't consider that a requirement, just nice to
have.

			Pekka