[PATCH] iommu: amd: Fix IOMMU perf counter clobbering during init

[Shuah Khan]

init_iommu_perf_ctr() clobbers the register when it checks write access
to IOMMU perf counters and fails to restore when they are writable.

Add save and restore to fix it.

Signed-off-by: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
---
 drivers/iommu/amd_iommu_init.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
index 568c52317757..c0ad4f293522 100644
--- a/drivers/iommu/amd_iommu_init.c
+++ b/drivers/iommu/amd_iommu_init.c
@@ -1655,27 +1655,37 @@ static int iommu_pc_get_set_reg(struct amd_iommu *iommu, u8 bank, u8 cntr,
 static void init_iommu_perf_ctr(struct amd_iommu *iommu)
 {
 	struct pci_dev *pdev = iommu->dev;
-	u64 val = 0xabcd, val2 = 0;
+	u64 val = 0xabcd, val2 = 0, save_reg = 0;
 
 	if (!iommu_feature(iommu, FEATURE_PC))
 		return;
 
 	amd_iommu_pc_present = true;
 
+	/* save the value to restore, if writable */
+	if (iommu_pc_get_set_reg(iommu, 0, 0, 0, &save_reg, false))
+		goto pc_false;
+
 	/* Check if the performance counters can be written to */
 	if ((iommu_pc_get_set_reg(iommu, 0, 0, 0, &val, true)) ||
 	    (iommu_pc_get_set_reg(iommu, 0, 0, 0, &val2, false)) ||
-	    (val != val2)) {
-		pci_err(pdev, "Unable to write to IOMMU perf counter.\n");
-		amd_iommu_pc_present = false;
-		return;
-	}
+	    (val != val2))
+		goto pc_false;
+
+	/* restore */
+	if (iommu_pc_get_set_reg(iommu, 0, 0, 0, &save_reg, true))
+		goto pc_false;
 
 	pci_info(pdev, "IOMMU performance counters supported\n");
 
 	val = readl(iommu->mmio_base + MMIO_CNTR_CONF_OFFSET);
 	iommu->max_banks = (u8) ((val >> 12) & 0x3f);
 	iommu->max_counters = (u8) ((val >> 7) & 0xf);
+
+pc_false:
+	pci_err(pdev, "Unable to read/write to IOMMU perf counter.\n");
+	amd_iommu_pc_present = false;
+	return;
 }
 
 static ssize_t amd_iommu_show_cap(struct device *dev,
-- 
2.20.1