Re: [PATCH kernel] KVM: PPC: Avoid mapping compound pages to TCEs in real mode

[Paul Mackerras <paulus@ozlabs.org>]

On Mon, Sep 03, 2018 at 01:28:44PM +1000, David Gibson wrote:
> On Fri, Aug 31, 2018 at 04:08:50PM +1000, Alexey Kardashevskiy wrote:
> > At the moment the real mode handler of H_PUT_TCE calls iommu_tce_xchg_rm()
> > which in turn reads the old TCE and if it was a valid entry - marks
> > the physical page dirty if it was mapped for writing. Since it is
> > the real mode, realmode_pfn_to_page() is used instead of pfn_to_page()
> > to get the page struct. However SetPageDirty() itself reads the compound
> > page head and returns a virtual address for the head page struct and
> > setting dirty bit for that kills the system.
> > 
> > This moves dirty bit setting before updating the hardware table
> 
> Um.. but now you're setting DIRTY based on the *new* TCE's
> permissions, instead of the old TCE's permissions, which I don't think
> is correct.

He's setting dirty on the page which is about to be mapped if the new
TCE allows the page to be written by the device.  Given that the page
is pinned, I don't see any reason why the dirty setting has to be done
when the page is unmapped rather than when it is mapped.  Do you see a
reason?

Paul.