[PATCH v2 2/2] iommu/amd-vi: do not zero IOMMU MMIO region

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

Attempting to memset the whole IOMMU MMIO region to zero is dangerous to
say the least.  We don't know what registers might be there, nor which
values might be safe for those registers.  On a forthcoming platform doing
the zeroing of the MMIO region does put the IOMMU in a broken state, which
is not recoverable by the IOMMU initialization procedure in Xen.

Instead attempt to forcefully disable the IOMMU ahead of enabling it.  Fold
map_iommu_mmio_region() into it's only caller, as the function body is just
an ioremap() call after the removal of the memset().

Fixes: 0700c962ac2d ("Add AMD IOMMU support into hypervisor")
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Zero the control register after calling disable_iommu().
 - Print a warning message if the IOMMU is handed enabled to Xen from
   firmware.
 - Fix commit log grammar issues.
---
 xen/drivers/passthrough/amd/iommu_init.c | 31 +++++++++++++-----------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/xen/drivers/passthrough/amd/iommu_init.c 
b/xen/drivers/passthrough/amd/iommu_init.c
index 76ae78e5ea53..ffc041211fb5 100644
--- a/xen/drivers/passthrough/amd/iommu_init.c
+++ b/xen/drivers/passthrough/amd/iommu_init.c
@@ -42,18 +42,6 @@ static bool iommu_has_ht_flag(struct amd_iommu *iommu, u8 
mask)
     return iommu->ht_flags & mask;
 }
 
-static int __init map_iommu_mmio_region(struct amd_iommu *iommu)
-{
-    iommu->mmio_base = ioremap(iommu->mmio_base_phys,
-                               IOMMU_MMIO_REGION_LENGTH);
-    if ( !iommu->mmio_base )
-        return -ENOMEM;
-
-    memset(iommu->mmio_base, 0, IOMMU_MMIO_REGION_LENGTH);
-
-    return 0;
-}
-
 static void __init unmap_iommu_mmio_region(struct amd_iommu *iommu)
 {
     if ( iommu->mmio_base )
@@ -1367,11 +1355,14 @@ static int __init amd_iommu_prepare_one(struct 
amd_iommu *iommu)
 {
     int rc = alloc_ivrs_mappings(iommu->sbdf.seg);
 
-    if ( !rc )
-        rc = map_iommu_mmio_region(iommu);
     if ( rc )
         return rc;
 
+    iommu->mmio_base = ioremap(iommu->mmio_base_phys,
+                               IOMMU_MMIO_REGION_LENGTH);
+    if ( !iommu->mmio_base )
+        return -ENOMEM;
+
     get_iommu_features(iommu);
 
     /*
@@ -1381,6 +1372,18 @@ static int __init amd_iommu_prepare_one(struct amd_iommu 
*iommu)
     if ( amd_iommu_max_paging_mode < amd_iommu_min_paging_mode )
         return -ERANGE;
 
+    /* Read current control register and forcefully disable the IOMMU. */
+    iommu->ctrl.raw = readq(iommu->mmio_base + IOMMU_CONTROL_MMIO_OFFSET);
+    if ( iommu->ctrl.iommu_en )
+        printk(XENLOG_WARNING
+               "AMD-Vi: IOMMU %pp enabled by firmware (%016lx)\n",
+               &iommu->sbdf, iommu->ctrl.raw);
+    disable_iommu(iommu, true);
+
+    /* With the IOMMU disabled zero the control register. */
+    iommu->ctrl.raw = 0;
+    writeq(0, iommu->mmio_base + IOMMU_CONTROL_MMIO_OFFSET);
+
     return 0;
 }
 
-- 
2.53.0