Re: [Xen-devel] [BUG] After upgrade to Xen 4.12.0 iommu=no-igfx

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

On Mon, Jul 22, 2019 at 01:54:00PM +0200, Paul Durrant wrote:
> > -----Original Message-----
> > From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > Sent: 22 July 2019 12:49
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Roman Shaposhnik' 
> > <roman@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; jgross@xxxxxxxx; Andrew Cooper 
> > <Andrew.Cooper3@xxxxxxxxxx>;
> > boris.ostrovsky@xxxxxxxxxx; jbeulich@xxxxxxxx
> > Subject: Re: [Xen-devel] [BUG] After upgrade to Xen 4.12.0 iommu=no-igfx
> > 
> > On Mon, Jul 22, 2019 at 08:20:36AM +0000, Paul Durrant wrote:
> > > > -----Original Message-----
> > > [snip]
> > > > > (XEN) Domain heap initialised
> > > > > (XEN) ACPI: 32/64X FACS address mismatch in FADT -
> > > > > 8ce8ef80/0000000000000000, using 32
> > > > > (XEN) IOAPIC[0]: apic_id 2, version 32, address 0xfec00000, GSI 0-119
> > > > > (XEN) Enabling APIC mode:  Flat.  Using 1 I/O APICs
> > > > > (XEN) [VT-D]  RMRR address range 8d800000..8fffffff not in reserved
> > > > > memory; need "iommu_inclusive_mapping=1"?
> > >
> > > This is your problem. In versions prior to 4.11 (I think, and certainly 
> > > 4.12)
> > iommu_inclusive_mapping used to default on, whereas now it appears to 
> > default off. In most
> > circumstances this is fine because there is a new flag, 
> > iommu_hwdom_reserved, which defaults on and
> > this makes sure that all e820 reserved regions are identity mapped (which 
> > usually covers undeclared
> > RMRRs). You have the opposite problem... a declared RMRR which is not 
> > reserved, so you will need
> > iommu_inclusive_mapping.
> > 
> > I think there's a bug in the initialization of iommu for a PV dom0,
> > which leaves dom0 without rmrr entries. Can you try the patch below
> > and report whether it solves your issue?
> > 
> > Thanks, Roger.
> > ---8<---
> > diff --git a/xen/drivers/passthrough/iommu.c 
> > b/xen/drivers/passthrough/iommu.c
> > index 79ec6719f5..9d91f0d633 100644
> > --- a/xen/drivers/passthrough/iommu.c
> > +++ b/xen/drivers/passthrough/iommu.c
> > @@ -185,7 +185,7 @@ void __hwdom_init iommu_hwdom_init(struct domain *d)
> >      register_keyhandler('o', &iommu_dump_p2m_table, "dump iommu p2m 
> > table", 0);
> > 
> >      hd->status = IOMMU_STATUS_initializing;
> > -    hd->need_sync = iommu_hwdom_strict && !iommu_use_hap_pt(d);
> > +    hd->need_sync = !iommu_use_hap_pt(d);
> 
> But this is going to mean the if() below is true for non-strict dom0, which 
> means it pointlessly maps the dom0 page list when hwdom_iommu_map() should 
> have already mapped all conventional RAM.

Right, this all seems quite broken. Non-translated guests (ie: PV)
would always need iommu page-table sync, but set_identity_p2m_entry
contains the following:

if ( !paging_mode_translate(p2m->domain) )
{
    if ( !need_iommu_pt_sync(d) )
        return 0;
    return iommu_legacy_map(d, _dfn(gfn_l), _mfn(gfn_l), PAGE_ORDER_4K,
                            IOMMUF_readable | IOMMUF_writable);
}

I wonder whether the usage of need_iommu_pt_sync is wrong there, and
should instead be !has_iommu_pt(d), since non-translated domains would
never share the iommu page-tables anyway.

In any case, I think need_sync must be set when the iommu page tables
are not shared, and gating it on iommu_hwdom_strict seems wrong to me,
the strictness of the iommu doesn't affect whether a sync is need or
not.

I've updated the patch to avoid the pointless mapping of dom0 page
list, but I haven't included the change to set_identity_p2m_entry.

Thanks, Roger.
---8<---
diff --git a/xen/drivers/passthrough/iommu.c b/xen/drivers/passthrough/iommu.c
index 79ec6719f5..abf9e38607 100644
--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -185,8 +185,8 @@ void __hwdom_init iommu_hwdom_init(struct domain *d)
     register_keyhandler('o', &iommu_dump_p2m_table, "dump iommu p2m table", 0);
 
     hd->status = IOMMU_STATUS_initializing;
-    hd->need_sync = iommu_hwdom_strict && !iommu_use_hap_pt(d);
-    if ( need_iommu_pt_sync(d) )
+    hd->need_sync = !iommu_use_hap_pt(d);
+    if ( iommu_hwdom_strict && need_iommu_pt_sync(d) )
     {
         struct page_info *page;
         unsigned int i = 0, flush_flags = 0;


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