RE: [PATCH] KVM: arm: Fix crash in free_hyp_pgds() if timer initialization fails

[Pavel Fedin <p.fedin@samsung.com>]

 Hello!

> > > Did you check if PAGE_HYP_DEVICE can mean something sane on a stage-2
> > > page table entry and vice verse?
> >
> >  I tried to, the chain of macros and variables is complicated enough not to
> > get 200% sure, but anyway PAGE_HYP_DEVICE (as well as PAGE_S2_DEVICE)
> > includes PROT_PTE_DEVICE, so this is definitely device.
> >  I even tried to construct some mask in order to make a single check for only
> > DEVICE flags, but, to make things even less understandable and predictable,
> > the same code with different bitfields is reused by ARM64. So, i thought that
> > it will be more reliable just to add a second test.
> 
> The thing I want to avoid is PAGE_HYP_DEVICE covering some normal S2
> mapping, which we *should* flush but that we now end up ignoring?  That
> doesn't sound like it can be the case because the device bit is the same
> bit for both types of page tables, correct?

 Yes, this is exactly what i think. If DEVICE bit is set, then it's somehow
device memory and it doesn't need flashing.

 Or, in order to be 200% sure, we could modify the whole unmapping logic to carry
over a flag, telling whether we are removing normal or HYP mappings. But wouldn't
this be much more complicated?

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia