On 15.10.2009, at 09:33, 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.
One thing I've been wondering for quite a while is that slot loop. Why
do we loop over all possible slots? Couldn't we just remember the max
extry (usually 1 or 2) and not loop MAX_SLOT_AMOUNT times?
That would be a really easy patch and give instant speed improvements
for everyone.
Alex
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