[xen master] x86/pvh: fix population of the low 1MB for dom0

[patchbot@xxxxxxx]

commit 2d5fc9120d556ec3c4b1acf0ab5660a6d3f7ebeb
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Mon Jan 24 17:13:12 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Jan 25 10:52:24 2022 +0000

    x86/pvh: fix population of the low 1MB for dom0
    
    RMRRs are setup ahead of populating the p2m and hence the ASSERT when
    populating the low 1MB needs to be relaxed when it finds an existing
    entry: it's either RAM or a RMRR resulting from the IOMMU setup.
    
    Rework the logic a bit and introduce a local mfn variable in order to
    assert that if the gfn is populated and not RAM it is an identity map.
    
    Fixes: 6b4f6a31ac ('x86/PVH: de-duplicate mappings for first Mb of Dom0 
memory')
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/dom0_build.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/dom0_build.c b/xen/arch/x86/hvm/dom0_build.c
index 43e1bf1248..8048987286 100644
--- a/xen/arch/x86/hvm/dom0_build.c
+++ b/xen/arch/x86/hvm/dom0_build.c
@@ -465,11 +465,16 @@ static int __init pvh_populate_p2m(struct domain *d)
     for ( i = rc = 0; i < MB1_PAGES; ++i )
     {
         p2m_type_t p2mt;
+        mfn_t mfn = get_gfn_query(d, i, &p2mt);
 
-        if ( mfn_eq(get_gfn_query(d, i, &p2mt), INVALID_MFN) )
+        if ( mfn_eq(mfn, INVALID_MFN) )
             rc = set_mmio_p2m_entry(d, _gfn(i), _mfn(i), PAGE_ORDER_4K);
         else
-            ASSERT(p2mt == p2m_ram_rw);
+            /*
+             * If the p2m entry is already set it must belong to a RMRR and
+             * already be identity mapped, or be a RAM region.
+             */
+            ASSERT(p2mt == p2m_ram_rw || mfn_eq(mfn, _mfn(i)));
         put_gfn(d, i);
         if ( rc )
         {
--
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