Re: [Xen-devel] Regression in RMRRs identity mapping for PVH Dom0

[Elena Ufimtseva <elena.ufimtseva@xxxxxxxxxx>]

On Thu, Sep 24, 2015 at 10:18:54AM +0100, Tim Deegan wrote:
> At 15:17 +0800 on 24 Sep (1443107852), Chen, Tiejun wrote:
> > On 9/23/2015 11:56 PM, Elena Ufimtseva wrote:
> > > Hi
> > >
> > > There is a regression in RMRR patch 
> > > 5ae03990c120a7b3067a52d9784c9aa72c0705a6 in
> > > new set_identity_p2m_entry. RMRRs are not being mapped in IOMMU for PVH 
> > > Dom0.
> > > This causes pages faults and some long 'hang-like' delays during boot and
> > > device assignments.
> > >
> > > During construct_dom0, in PVH path  p2m is being constructed and identity 
> > > mapped
> > > in IOMMU. The p2m type is p2m_mmio_direct and p2m access p2m_rwx.
> > > New code used to map RMRRs invoked from rmrr_identity_mapping
> > > checks if p2m entry exists with same type and access and if yes, skips 
> > > iommu
> > > mapping. Since there are p2m entries for pvh dom0 iomem, RMRRs are not 
> > > being
> > > mapped in IOMMU.
> > >
> > > This debug patch attached fixes this and Ill be glad to see if there is a 
> > > more elegant fix.
> > 
> > Based on your explanation, sounds pvh always creates this mapping 
> > beforehand, so what about this?
> > 
> > diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> > index cf8485e..d026845 100644
> > --- a/xen/arch/x86/mm/p2m.c
> > +++ b/xen/arch/x86/mm/p2m.c
> > @@ -964,7 +964,7 @@ int set_identity_p2m_entry(struct domain *d, 
> > unsigned long gfn,
> >       struct p2m_domain *p2m = p2m_get_hostp2m(d);
> >       int ret;
> > 
> > -    if ( !paging_mode_translate(p2m->domain) )
> > +    if ( !paging_mode_translate(p2m->domain) || is_pvh_domain(d) )
> 
> Sorry, but that wouldn't be safe. :(  PVH domains need the same
> protection as any other paging_mode_translate ones.
> 
> AIUI the problem is that before the call to set_identity_p2m_entry(),
> PVH dom0 has a p2m entry covering this range but no IOMMU entry.  Is
> that right?  So the fix will be to make PVH dom0 construction set up
> the IOMMU correctly when it sets up the p2m.

Yes, thats right. Rework of construct_dom0 and its PVH part should help.
> 
> Cheers,
> 
> Tim.

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