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

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:

quoted

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.