Re: [Xen-devel] [xen-unstable test] 56456: regressions - FAIL

[Tim Deegan <tim@xxxxxxx>]

At 12:50 +0200 on 18 May (1431953450), Roger Pau Monné wrote:
> El 18/05/15 a les 12.17, Tim Deegan ha escrit:
> > At 09:34 +0100 on 18 May (1431941676), Jan Beulich wrote:
> >>>>> On 16.05.15 at 13:45, <roger.pau@xxxxxxxxxx> wrote:
> >>> El 16/05/15 a les 10.51, osstest service user ha escrit:
> >>>> flight 56456 xen-unstable real [real]
> >>>> http://logs.test-lab.xenproject.org/osstest/logs/56456/
> >>>>
> >>>> Regressions :-(
> >>>
> >>> This is my fault, paging_gva_to_gfn cannot be used to translate a PV
> >>> guest VA to a GFN. The patch above restores the previous path for PV
> >>> callers.
> >>
> >> While Tim would have the final say, I certainly would prefer to revert
> >> the offending patch and then apply a correct new version in its stead
> >> in this case (where the fix is not a simple, few lines change).
> > 
> > I would be OK with a follow-up fix here, but I'm not convinced that
> > this is it.
> > 
> > In particular, paging_mode_enabled() should be true for any PV domain
> > that's in log-dirty mode, so presumably the failure is only for lgd
> > ops on VMs that don't have lgd enabled.  So maybe we can either:
> >  - return an error for that case (but we'd want to understand how we
> >    got there first); or
> 
> The error is caused because we are trying to use paging_gva_to_gfn
> against a Dom0 VA, which is a PV guest.

Oh right; I had got confused.

> >  - have map_dirty_bitmap() DTRT, with something like access_ok() +
> >    a linear-pagetable lookup to find the frame.
> 
> That was my first intention, but AFAICT we have no function in tree to
> resolve a PV guest VA into a GFN/MFN. The closest thing I could find was
> using guest_walk_tables + guest_walk_to_gfn in order to obtain the gfn.
> Should I send a patch to introduce a pv_gva_to_gfn function based on that?

PV guest addresses need a bit more care: we need to at least call
access_ok() on them to make sure we're not going to write to a Xen
address.  For the translation itself, we could use guest_walk_tables(),
though I wouldn't be surprised if that only works on
paging_mode_enabled() guests too.  Worth a try though.  Otherwise,
guest_get_eff_l1e() will get the gfn but doesn't include a check for
write permissions, so might introduce an in-guest issue.

Tim.

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