linux-kernel - [PATCH v4 4/4] iommu/amd: Skip enabling command/event buffers for kdump

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Message-ID: <eeecb08fee542b36713769a16e31532537de0727.1753133022.git.ashish.kalra@amd.com>
Date: Mon, 21 Jul 2025 21:53:38 +0000
From: Ashish Kalra <Ashish.Kalra@....com>
To: <joro@...tes.org>, <suravee.suthikulpanit@....com>,
	<thomas.lendacky@....com>, <Sairaj.ArunKodilkar@....com>,
	<Vasant.Hegde@....com>, <herbert@...dor.apana.org.au>
CC: <seanjc@...gle.com>, <pbonzini@...hat.com>, <will@...nel.org>,
	<robin.murphy@....com>, <john.allen@....com>, <davem@...emloft.net>,
	<michael.roth@....com>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <linux-crypto@...r.kernel.org>,
	<kvm@...r.kernel.org>
Subject: [PATCH v4 4/4] iommu/amd: Skip enabling command/event buffers for kdump

From: Ashish Kalra <ashish.kalra@....com>

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.

Signed-off-by: Ashish Kalra <ashish.kalra@....com>
---
 drivers/iommu/amd/init.c | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index 4ba2281e06f2..8a2e3e96fe69 100644
--- a/drivers/iommu/amd/init.c
+++ b/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);
-- 
2.34.1