Re: [Qemu-devel] [PATCH 6/6] kvm: Fix dirty tracking with large kernel page size

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Sun, 2012-03-04 at 12:49 +0200, Avi Kivity wrote:
> On 02/28/2012 11:48 PM, Benjamin Herrenschmidt wrote:
> > On Tue, 2012-02-28 at 14:32 +0200, Avi Kivity wrote:
> >
> > > What if TARGET_PAGE_SIZE > getpagesize()?  Or is that impossible?
> >
> > We have yet to encounter such a case. It's not currently possible on
> > power (some old embedded chips could do 1K and 2K page sizes in the TLB
> > iirc but we never supported that in Linux and it's being phased out in
> > HW).
> >
> > I suggest that gets dealt with when/if it needs to, which means probably
> > never :-)
> 
> Doesn't ppc support both 4k and 64k pages?  Suppose you run a 4k guest
> on a 64k host?
> 
> Maybe I'm misremembering or misunderstanding something.

TARGET_PAGE_SIZE in qemu is always 4k for powerpc, it's a compile time
#define.

The host kernel exposes the dirty bit map with a bit per -host- kernel
PAGE_SIZE (which is what getpagesize() gets you in qemu).

My patch makes thus makes things work when the host uses 64K page sizes.
In all cases, the guest page size is irrelevant, this is purely a
problem between the host kernel and qemu.

Cheers,
Ben.