[Xen-devel] [PATCH RFC] xen: allow guest_remove_page to remove p2m_mmio_direct pages

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

IF a guest tries to do a foreign/grant mapping in a memory region
marked as p2m_mmio_direct Xen will complain with the following
message:

(XEN) memory.c:241:d0v0 Bad page free for domain 0

Albeit the mapping will succeed. This is specially problematic for PVH
Dom0, in which we map all the e820 holes and memory up to 4GB as
p2m_mmio_direct.

In order to deal with it, add a special casing for p2m_mmio_direct
regions in guest_remove_page if the domain is a hardware domain, that
calls clear_mmio_p2m_entry in order to remove the mappings.

Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
---
I've spoken with Jan about this issue on IRC, and he felt that adding
such a special casing in guest_remove_page is not the right thing to
do, I could look into two other approaches at least:

 - Make XENMEM_remove_from_physmap able to remove p2m_mmio_direct
   ranges, right now it doesn't work with those ranges because
   get_page_from_gfn fails when used on p2m_mmio_direct p2m entries.

 - Extend HVMOP_set_mem_type to be able to perform the conversion from
   p2m_mmio_direct to p2m_invalid and vice versa on Dom0 request.

I have to admit I don't like any of these two options, because it
would mean we will need to perform another hypercall before mapping,
which will make it even slower.
---
 xen/common/memory.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/xen/common/memory.c b/xen/common/memory.c
index 257f4b0..ecd3786 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -204,6 +204,12 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
         p2m_mem_paging_drop_page(d, gmfn, p2mt);
         return 1;
     }
+    if ( is_hardware_domain(d) && p2mt == p2m_mmio_direct )
+    {
+        clear_mmio_p2m_entry(d, gmfn);
+        put_gfn(d, gmfn);
+        return 1;
+    }
 #else
     mfn = gmfn_to_mfn(d, gmfn);
 #endif
-- 
1.7.7.5 (Apple Git-26)


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