Re: [PATCH 6/6 v2] kvm: powerpc: use caching attributes as per linux pte

[Scott Wood <scottwood@freescale.com>]

On Sat, 2013-08-03 at 14:25 +1000, Benjamin Herrenschmidt wrote:
> On Sat, 2013-08-03 at 03:11 +0000, Bhushan Bharat-R65777 wrote:
> > 
> > > 
> > > Could you explain why we need to set dirty/referenced on the PTE, when we didn't
> > > need to do that before? All we're getting from the PTE is wimg.
> > > We have MMU notifiers to take care of the page being unmapped, and we've already
> > > marked the page itself as dirty if the TLB entry is writeable.
> > 
> > I pulled this code from book3s.
> > 
> > Ben, can you describe why we need this on book3s ?
> 
> If you let the guest write to the page you must set the dirty bit on the PTE
> (or the struct page, at least one of them), similar with accessed on any access.
> 
> If you don't, the VM might swap the page out without writing it back to disk
> for example, assuming it contains no modified data.

We've already marked the page itself as dirty using kvm_set_pfn_dirty(),
and if the VM swaps it out we'll get an MMU notifier callback.  If we
marked the PTE dirty/accessed instead, is there any guarantee it will
stay marked dirty/accessed until the next MMU notifier?

-Scott