Re: [PATCH 4/5] KVM: PPC: Book3s HV: Implement get_dirty_log using hardware changed bit

[Paul Mackerras <paulus@samba.org>]

On Fri, Dec 23, 2011 at 02:23:30PM +0100, Alexander Graf wrote:

> So if I read things correctly, this is the only case you're setting
> pages as dirty. What if you have the following:
> 
>   guest adds HTAB entry x
>   guest writes to page mapped by x
>   guest removes HTAB entry x
>   host fetches dirty log

In that case the dirtiness is preserved in the setting of the
KVMPPC_RMAP_CHANGED bit in the rmap entry.  kvm_test_clear_dirty()
returns 1 if that bit is set (and clears it).  Using the rmap entry
for this is convenient because (a) we also use it for saving the
referenced bit when a HTAB entry is removed, and we can transfer both
R and C over in one operation; (b) we need to be able to save away the
C bit in real mode, and we already need to get the real-mode address
of the rmap entry -- if we wanted to save it in a dirty bitmap we'd
have to do an extra translation to get the real-mode address of the
dirty bitmap word; (c) to avoid SMP races, if we were asynchronously
setting bits in the dirty bitmap we'd have to do the double-buffering
thing that x86 does, which seems more complicated than using the rmap
entry (which we already have a lock bit for).

> PS: Always CC kvm@vger for stuff that other might want to review
> (basically all patches)

So why do we have a separate kvm-ppc list then? :)

Paul.