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[Xen-devel] [PATCH] AMD/guest_iommu: Disable guest iommu support

AMD Guest IOMMU support was added to allow correct use of PASID and PRI
hardware support with an ATS-aware guest driver.

However, support cannot possibly function as guest_iommu_set_base() has no
callers.  This means that its MMIO region's P2M pages are not set to
p2m_mmio_dm, preventing any invocation of the MMIO read/write handlers.

c/s fd186384 "x86/HVM: extend LAPIC shortcuts around P2M lookups" introduces a
path (via hvm_mmio_internal()) where iommu_mmio_handler claims its MMIO range,
and causes __hvm_copy() to fail with HVMCOPY_bad_gfn_to_mfn.

iommu->mmio_base defaults to 0, with a range of 8 pages, and is unilaterally
enabled in any HVM guests when the host IOMMU(s) supports any extended

Unfortunately, HVMLoader's AP boot trampoline executes an `lmsw` instruction
at linear address 0x100c which unconditionally requires emulation.  The
instruction fetch in turn fails as __hvm_copy() fails with

The result is that multi-vcpu HVM guests do not work on newer AMD hardware, if
IOMMU support is enabled in the BIOS.

Change the default mmio_base address to ~0ULL.  This prevents
guest_iommu_mmio_range() from actually claiming any physical range
whatsoever, which allows the emulation of `lmsw` to succeed.

Reported-by: Roberto Luongo <rluongo@xxxxxxxx>
Suggested-by: Jan Beulich <JBeulich@xxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Tested-by: Roberto Luongo <rluongo@xxxxxxxx>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
CC: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@xxxxxxx>
 xen/drivers/passthrough/amd/iommu_guest.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/drivers/passthrough/amd/iommu_guest.c 
index 5660020..98e7b38 100644
--- a/xen/drivers/passthrough/amd/iommu_guest.c
+++ b/xen/drivers/passthrough/amd/iommu_guest.c
@@ -885,6 +885,7 @@ int guest_iommu_init(struct domain* d)
+    iommu->mmio_base = ~0ULL;
     iommu->domain = d;
     hd->arch.g_iommu = iommu;

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