Re: [Xen-devel] [PATCH] p2m/ept: pass correct level to atomic_write_ept_entry in ept_invalidate_emt

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Aug 27, 2019 at 05:23:33PM +0200, Jan Beulich wrote:
> On 23.08.2019 07:58,  Tian, Kevin  wrote:
> > > From: Roger Pau Monne [mailto:roger.pau@xxxxxxxxxx]
> > > Sent: Tuesday, August 20, 2019 11:38 PM
> > > 
> > > The level passed to ept_invalidate_emt corresponds to the EPT entry
> > > passed as the mfn parameter, which is a pointer to an EPT page table,
> > > hence the entries in that page table will have one level less than the
> > > parent.
> > > 
> > > Fix the call to atomic_write_ept_entry to pass the correct level, ie:
> > > one level less than the parent.
> > > 
> > > Fixes: 50fe6e73705 ('pvh dom0: add and remove foreign pages')
> > > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > > ---
> > > Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> > > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> > > Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Cc: Wei Liu <wl@xxxxxxx>
> > > Cc: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> > > Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > > ---
> > >   xen/arch/x86/mm/p2m-ept.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
> > > index 6b8468c793..23ae6e9da3 100644
> > > --- a/xen/arch/x86/mm/p2m-ept.c
> > > +++ b/xen/arch/x86/mm/p2m-ept.c
> > > @@ -367,7 +367,7 @@ static bool_t ept_invalidate_emt(struct p2m_domain
> > > *p2m, mfn_t mfn,
> > >           e.emt = MTRR_NUM_TYPES;
> > >           if ( recalc )
> > >               e.recalc = 1;
> > > -        rc = atomic_write_ept_entry(p2m, &epte[i], e, level);
> > > +        rc = atomic_write_ept_entry(p2m, &epte[i], e, level - 1);
> > >           ASSERT(rc == 0);
> > >           changed = 1;
> > >       }
> > 
> > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>.
> > 
> > One small comment about readability. What about renaming 'level'
> > to 'parent_level' in this function?
> 
> And also taking the opportunity and making it unsigned int?

I've been thinking about this, and I'm not sure renaming level to
parent_level is correct, level actually matches the level of the mfn
also passed as a parameter, and hence it's correct. It's the function
itself that actually iterates over the page table entries, and hence
descends one level into the page tables.

ie: I could add something like ASSERT(level) to make sure no level 0
entries are passed to the function, but renaming doesn't seem correct
to me.

Thanks, Roger.

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