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

[Avi Kivity <avi@redhat.com>]

On 03/04/2012 01:53 PM, Benjamin Herrenschmidt wrote:
> 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.

Right (and I actually knew all this stuff before :( ).

-- 
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