[PATCH v2 2.5/4] xen/x86: Replace mandatory barriers with compiler barriers

[Andrew Cooper <andrew.cooper3@citrix.com>]

In this case, rmb() is being used for its compiler barrier property.  Replace
it with an explicit barrer() and comment, to avoid it becoming an unnecessary
lfence instruction (when rmb() gets fixed) or looking like an SMP issue.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
---
 xen/drivers/passthrough/amd/iommu_init.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/drivers/passthrough/amd/iommu_init.c b/xen/drivers/passthrough/amd/iommu_init.c
index a459e99..474992a 100644
--- a/xen/drivers/passthrough/amd/iommu_init.c
+++ b/xen/drivers/passthrough/amd/iommu_init.c
@@ -558,7 +558,7 @@ static void parse_event_log_entry(struct amd_iommu *iommu, u32 entry[])
             return;
         }
         udelay(1);
-        rmb();
+        barrier(); /* Prevent hoisting of the entry[] read. */
         code = get_field_from_reg_u32(entry[1], IOMMU_EVENT_CODE_MASK,
                                       IOMMU_EVENT_CODE_SHIFT);
     }
@@ -663,7 +663,7 @@ void parse_ppr_log_entry(struct amd_iommu *iommu, u32 entry[])
             return;
         }
         udelay(1);
-        rmb();
+        barrier(); /* Prevent hoisting of the entry[] read. */
         code = get_field_from_reg_u32(entry[1], IOMMU_PPR_LOG_CODE_MASK,
                                       IOMMU_PPR_LOG_CODE_SHIFT);
     }
-- 
2.1.4


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