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Re: [Qemu-devel] [PATCH v4 03/11] dataplane: add host memory mapping cod
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v4 03/11] dataplane: add host memory mapping code |
Date: |
Thu, 29 Nov 2012 15:26:56 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Thu, Nov 29, 2012 at 03:54:25PM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 22, 2012 at 04:16:44PM +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.
> >
> > When the physical memory map changes the MemoryListener callbacks are
> > invoked. They build up a new list of memory regions which is finally
> > installed when the list has been completed.
> >
> > Note that this approach is not safe across memory hotplug because mapped
> > pointers may still be in used across memory unplug. However, this is
> > currently a problem for QEMU in general and needs to be addressed in the
> > future.
> >
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
>
> Worth bothering with binary search?
> vhost does a linear search over regions because
> the number of ram regions is very small.
memory.c does binary search. I did the same but in practice there are
<20 regions for a simple VM. It's probably not worth it but without
performance results this is speculation.
I think there's no harm in using binary search to start with.
> > +static void hostmem_listener_append_region(MemoryListener *listener,
> > + MemoryRegionSection *section)
> > +{
> > + Hostmem *hostmem = container_of(listener, Hostmem, listener);
> > +
> > + if (memory_region_is_ram(section->mr)) {
> > + hostmem_append_new_region(hostmem, section);
> > + }
>
> I think you also need to remove VGA region since you
> don't mark these pages as dirty so access there won't work.
I don't understand. If memory in the VGA region returns true from
memory_region_is_ram(), why would there be a problem?
[Qemu-devel] [PATCH v4 01/11] raw-posix: add raw_get_aio_fd() for virtio-blk-data-plane, Stefan Hajnoczi, 2012/11/22
[Qemu-devel] [PATCH v4 04/11] dataplane: add virtqueue vring code, Stefan Hajnoczi, 2012/11/22
[Qemu-devel] [PATCH v4 05/11] dataplane: add event loop, Stefan Hajnoczi, 2012/11/22
[Qemu-devel] [PATCH v4 07/11] iov: add iov_discard() to remove data, Stefan Hajnoczi, 2012/11/22
[Qemu-devel] [PATCH v4 06/11] dataplane: add Linux AIO request queue, Stefan Hajnoczi, 2012/11/22