Re: [Qemu-devel] [PATCH v6 03/12] dataplane: add host memory mapping code

[Michael S. Tsirkin]

On Wed, Dec 12, 2012 at 04:34:21PM +0100, Stefan Hajnoczi wrote:
> On Tue, Dec 11, 2012 at 08:09:56PM +0200, Michael S. Tsirkin wrote:
> > On Tue, Dec 11, 2012 at 10:32:28AM -0600, Anthony Liguori wrote:
> > > "Michael S. Tsirkin" <address@hidden> writes:
> > > 
> > > > On Tue, Dec 11, 2012 at 04:27:49PM +0100, Stefan Hajnoczi wrote:
> > > >> On Tue, Dec 11, 2012 at 3:13 PM, Michael S. Tsirkin <address@hidden> 
> > > >> wrote:
> > > >> > On Mon, Dec 10, 2012 at 02:09:36PM +0100, Stefan Hajnoczi wrote:
> > > >> >> The data plane thread needs to map guest physical addresses to host
> > > >> >> pointers.  Normally this is done with cpu_physical_memory_map() but 
> > > >> >> the
> > > >> >> function assumes the global mutex is held.  The data plane thread 
> > > >> >> does
> > > >> >> not touch the global mutex and therefore needs a thread-safe memory
> > > >> >> mapping mechanism.
> > > >> >>
> > > >> >> Hostmem registers a MemoryListener similar to how vhost collects and
> > > >> >> pushes memory region information into the kernel.  There is a
> > > >> >> fine-grained lock on the regions list which is held during lookup 
> > > >> >> and
> > > >> >> when installing a new regions list.
> > > >> >
> > > >> > Can we export and reuse the vhost code for this?
> > > >> > I think you will find this advantageous when you add migration
> > > >> > support down the line.
> > > >> > And if you find it necessary to use MemoryListener e.g. for 
> > > >> > performance
> > > >> > reasons, then vhost will likely benefit too.
> > > >> 
> > > >> It's technically possible and not hard to do but it prevents
> > > >> integrating deeper with core QEMU as the memory API becomes
> > > >> thread-safe.
> > > >> 
> > > >> There are two ways to implement dirty logging:
> > > >> 1. The vhost log approach which syncs dirty information periodically.
> > > >> 2. A cheap thread-safe way to mark dirty outside the global mutex,
> > > >> i.e. a thread-safe memory_region_set_dirty().
> > > >
> > > > You don't normally want to dirty the whole region,
> > > > you want to do this to individual pages.
> > > >
> > > >> If we can get thread-safe guest memory load/store in QEMU then #2 is
> > > >> included.  We can switch to using hw/virtio.c instead of
> > > >> hw/dataplane/vring.c, we get dirty logging for free, we can drop
> > > >> hostmem.c completely, etc.
> > > >> 
> > > >> Stefan
> > > >
> > > > So why not reuse existing code? If you drop it later it won't
> > > > matter what you used ...
> > > 
> > > Let's not lose sight of the forest for the trees here...
> > > 
> > > This whole series is not reusing existing code.  That's really the whole
> > > point.
> > > 
> > > The point is to take the code (duplication and all) and then do all of
> > > the refactoring to use common code in the tree itself.
> > > 
> > > If we want to put this in a hw/staging/ directory, that's fine by me
> > > too.
> > > 
> > > Regards,
> > > 
> > > Anthony Liguori
> > 
> > Yes I agree. I think lack of handling for cross regin descriptors
> > bothers me a bit more.
> 
> The two things you've mentioned both aren't handled by hw/virtio.c:
> 
> 1. Issue: Indirect descriptors have no alignment restrictions and can
>    cross regions.
> 
>    hw/virtio.c uses vring_desc_flags() and other accessor functions,
>    which do lduw_phys() - there is no memory region boundary checking
>    here.

Since addresses are aligned this one is fine I think.

> 2. Issue: Virtio buffers can cross memory region boundaries.
> 
>    hw/virtio.c maps buffers 1:1 using virtqueue_map_sg() and exits if
>    mapping fails.  It does not split buffers if they cross a memory
>    region.
> 
> These are definitely ugly corner cases but hw/virtio.c is proof that
> we're not hitting them in practice.
> 
> Stefan

Yes, this one seems ugly. Maybe add a TODO?

OK let's assume we want to put it in staging/
I worry about the virtio-blk changes being isolated.
Can you put ifdef CONFIG_VIRTIO_BLK_DATA_PLANE around
them all to avoid dependency on that header completely
if configured out?


-- 
MST