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

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Message-Id: <20121204234829.65472b5ec4aedcf86cccc05f@gmail.com>
Date:	Tue, 4 Dec 2012 23:48:29 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	mtosatti@...hat.com, kvm@...r.kernel.org, gleb@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/6] kvm: Growable memory slot array

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

Thanks,
	Takuya
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