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

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Message-ID: <1354634801.1809.411.camel@bling.home>
Date:	Tue, 04 Dec 2012 08:26:41 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.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 Tue, 2012-12-04 at 23:48 +0900, Takuya Yoshikawa wrote:
> 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.

Great!  A bi-modal bitmap would be rather ugly, so I look forward to
your patches to remove it entirely ;)  Thanks,

Alex

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