Re: [RFC PATCH 0/6] kvm: Growable memory slot array

From: Takuya Yoshikawa
Date: Tue Dec 04 2012 - 09:48:29 EST

On Mon, 03 Dec 2012 16:39:05 -0700
Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:

> A couple notes/questions; in the previous version we had a
> kvm_arch_flush_shadow() call when we increased the number of slots.
> I'm not sure if this is still necessary. I had also made the x86
> specific slot_bitmap dynamically grow as well and switch between a
> direct bitmap and indirect pointer to a bitmap. That may have
> contributed to needing the flush. I haven't done that yet here
> because it seems like an unnecessary complication if we have a max
> on the order of 512 or 1024 entries. A bit per slot isn't a lot of
> overhead. If we want to go more, maybe we should make it switch.

I have a patch set which removes the slot_bitmap in kvm mmu page
by using reverse mappings for write protecting a memslot.

A bit of concern I still have is the total write protection time
for large memslots. But since this approach allows us to control
mmu_lock hold time, I think this is a reasonable trade-off.
... and this should be much better than introducing any complication
for slot_bitmap handling.

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