iommu/amd: Skip enabling command/event buffers for kdump

After a panic if SNP is enabled in the previous kernel then the kdump
kernel boots with IOMMU SNP enforcement still enabled.

IOMMU command buffers and event buffer registers remain locked and
exclusive to the previous kernel. Attempts to enable command and event
buffers in the kdump kernel will fail, as hardware ignores writes to
the locked MMIO registers as per AMD IOMMU spec Section 2.12.2.1.

Skip enabling command buffers and event buffers for kdump boot as they
are already enabled in the previous kernel.

Reviewed-by: Vasant Hegde <[email protected]>
Tested-by: Sairaj Kodilkar <[email protected]>
Signed-off-by: Ashish Kalra <[email protected]>
Link: https://lore.kernel.org/r/576445eb4f168b467b0fc789079b650ca7c5b037.1756157913.git.ashish.kalra@amd.com
Signed-off-by: Joerg Roedel <[email protected]>

Author: ashkalra

drivers/iommu/amd/init.c

@@ -821,11 +821,16 @@ static void iommu_enable_command_buffer(struct amd_iommu *iommu)
 
 	BUG_ON(iommu->cmd_buf == NULL);
 
-	entry = iommu_virt_to_phys(iommu->cmd_buf);
-	entry |= MMIO_CMD_SIZE_512;
-
-	memcpy_toio(iommu->mmio_base + MMIO_CMD_BUF_OFFSET,
-		    &entry, sizeof(entry));
+	if (!is_kdump_kernel()) {
+		/*
+		 * Command buffer is re-used for kdump kernel and setting
+		 * of MMIO register is not required.
+		 */
+		entry = iommu_virt_to_phys(iommu->cmd_buf);
+		entry |= MMIO_CMD_SIZE_512;
+		memcpy_toio(iommu->mmio_base + MMIO_CMD_BUF_OFFSET,
+			    &entry, sizeof(entry));
+	}
 
 	amd_iommu_reset_cmd_buffer(iommu);
 }
@@ -876,10 +881,15 @@ static void iommu_enable_event_buffer(struct amd_iommu *iommu)
 
 	BUG_ON(iommu->evt_buf == NULL);
 
-	entry = iommu_virt_to_phys(iommu->evt_buf) | EVT_LEN_MASK;
-
-	memcpy_toio(iommu->mmio_base + MMIO_EVT_BUF_OFFSET,
-		    &entry, sizeof(entry));
+	if (!is_kdump_kernel()) {
+		/*
+		 * Event buffer is re-used for kdump kernel and setting
+		 * of MMIO register is not required.
+		 */
+		entry = iommu_virt_to_phys(iommu->evt_buf) | EVT_LEN_MASK;
+		memcpy_toio(iommu->mmio_base + MMIO_EVT_BUF_OFFSET,
+			    &entry, sizeof(entry));
+	}
 
 	/* set head and tail to zero manually */
 	writel(0x00, iommu->mmio_base + MMIO_EVT_HEAD_OFFSET);