On 10/16/2009 04:46 AM, Marcelo Tosatti wrote:
On Thu, Oct 15, 2009 at 04:33:11PM +0900, Avi Kivity wrote:
One way to improve the gfn_to_pfn() memslot search is to register just
one slot. This can only work on 64-bit, since even the smallest guests
need 4GB of physical address space. Apart from speeding up
gfn_to_page(), it would also speed up mmio which must iterate over all
slots, so a lookup cache cannot help.
This would require quite a bunch of changes:
- modify gfn_to_pfn() to fail gracefully if the page is in the slot but
unmapped (hole handling)
- modify qemu to reserve the guest physical address space
- modify qemu memory allocation to use MAP_FIXED to allocate memory
- some hack for the vga aliases (mmap an fd multiple times?)
- some hack for the vmx-specific pages (e.g. APIC-access page)
Not sure it's worthwhile, but something to keep in mind if a simple
cache or sort by size is insufficient due to mmio.
Downside is you lose the ability to write protect a small slot only
(could mprotect(MAP_READ) the desired area but get_log+write_protect
must be atomic).
Also if you enable dirty log for the large slot largepages are disabled.
I guess that shoots this idea down. We could perhaps only enable it if
a vnc client is not connected and we don't track vga updates.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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