[PATCH v6 7/7] iommu/amd: Provide kernel command line option to enable GAPPI
From: Sairaj Kodilkar
Date: Thu Oct 22 2026 - 10:27:40 EST
Enable GAPPI when the IOMMU GAPPISup extended feature bit is set and
the kernel is booted with the amd_iommu=gappi kernel parameter.
Document the option in Documentation/admin-guide/kernel-parameters.txt
and mechanism in Documentation/arch/x86/iommu.rst.
Co-developed-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
Signed-off-by: Sairaj Kodilkar <sarunkod@xxxxxxx>
---
.../admin-guide/kernel-parameters.txt | 3 +-
Documentation/arch/x86/iommu.rst | 22 ++++++++++++
drivers/iommu/amd/amd_iommu_types.h | 2 ++
drivers/iommu/amd/init.c | 34 ++++++++++++++++---
4 files changed, 56 insertions(+), 5 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index e75344f4e0cd..b260ebef118f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -456,7 +456,8 @@ Kernel parameters
to 4 KiB.
v2_pgsizes_only - Limit page-sizes used for v1 page-tables
to 4KiB/2Mib/1GiB.
-
+ gappi - Use GAPPI instead of GALOG for vCPU
+ wakeup notification.
amd_iommu_dump= [HW,X86-64]
Enable AMD IOMMU driver option to dump the ACPI table
diff --git a/Documentation/arch/x86/iommu.rst b/Documentation/arch/x86/iommu.rst
index 79c33560299b..6bac30f5abbd 100644
--- a/Documentation/arch/x86/iommu.rst
+++ b/Documentation/arch/x86/iommu.rst
@@ -150,3 +150,25 @@ AMD Fault reporting
AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0007 address=0xffffc02000 flags=0x0000]
AMD-Vi: Event logged [IO_PAGE_FAULT device=07:00.0 domain=0x0007 address=0xffffc02000 flags=0x0000]
+
+
+AMD vCPU wakeup Mechanism
+--------------------------
+
+GALOG
+^^^^^
+
+GALOG is a legacy notification path to wakeup the vCPU waiting on an interrupt.
+When IOMMU receives an interrupt destined to vCPU which is scheduled out, it
+updates the vAPIC backing page IRR and appends vCPU tags to a shared GA log
+buffer and raises a single GA log interrupt. IOMMU driver scans this buffer to
+decide which vCPU to wakeup.
+
+GAPPI
+^^^^^
+
+GAPPI is an alternative to GALOG. With GAPPI enabled, the IOMMU still updates
+the guest vAPIC backing page IRR, but may deliver a physical APIC interrupt
+directly to IRTE[Destination], using IRTE[GATag][7:0] as the vector. This
+distributes host wakeup notifications across CPUs instead of centralizing them
+in a log buffer. This can be enabled by adding option amd_iommu=gappi
diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h
index b89a2b3df0a4..51edd617ddca 100644
--- a/drivers/iommu/amd/amd_iommu_types.h
+++ b/drivers/iommu/amd/amd_iommu_types.h
@@ -78,6 +78,7 @@
#define FEATURE_X2APIC BIT_ULL(2)
#define FEATURE_NX BIT_ULL(3)
#define FEATURE_GT BIT_ULL(4)
+#define FEATURE_GAPPI BIT_ULL(5)
#define FEATURE_IA BIT_ULL(6)
#define FEATURE_GA BIT_ULL(7)
#define FEATURE_HE BIT_ULL(8)
@@ -178,6 +179,7 @@
#define CONTROL_EPH_EN 45
#define CONTROL_XT_EN 50
#define CONTROL_INTCAPXT_EN 51
+#define CONTROL_GAPPI_EN 55
#define CONTROL_GCR3TRPMODE 58
#define CONTROL_IRTCACHEDIS 59
#define CONTROL_SNPAVIC_EN 61
diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index 78e3c14dc1c2..72394c92a3fa 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -448,6 +448,9 @@ static void iommu_disable(struct amd_iommu *iommu)
iommu_feature_disable(iommu, CONTROL_GALOG_EN);
iommu_feature_disable(iommu, CONTROL_GAINT_EN);
+ /* Disable IOMMU GAPPI */
+ iommu_feature_disable(iommu, CONTROL_GAPPI_EN);
+
/* Disable IOMMU PPR logging */
iommu_feature_disable(iommu, CONTROL_PPRLOG_EN);
iommu_feature_disable(iommu, CONTROL_PPRINT_EN);
@@ -2273,7 +2276,7 @@ static void print_iommu_info(void)
{
int i;
static const char * const feat_str[] = {
- "PreF", "PPR", "X2APIC", "NX", "GT", "[5]",
+ "PreF", "PPR", "X2APIC", "NX", "GT", "GAPPI",
"IA", "GA", "HE", "PC"
};
@@ -2986,6 +2989,13 @@ static void enable_iommus_vapic(void)
struct amd_iommu *iommu;
for_each_iommu(iommu) {
+ /*
+ * Disable GAPPI, do not check amd_iommu_gappi as it may be
+ * false in new kexec kernel even though previous kernel has
+ * enabled it.
+ */
+ iommu_feature_disable(iommu, CONTROL_GAPPI_EN);
+
/*
* Disable GALog if already running. It could have been enabled
* in the previous boot before kdump.
@@ -3027,10 +3037,19 @@ static void enable_iommus_vapic(void)
return;
}
+ if (amd_iommu_gappi &&
+ !(check_feature(FEATURE_GAPPI) &&
+ AMD_IOMMU_GUEST_IR_VAPIC(amd_iommu_guest_ir))) {
+ pr_warn("GAPPI is not supported.\n");
+ amd_iommu_gappi = false;
+ }
+
/* Enabling GAM and SNPAVIC support */
for_each_iommu(iommu) {
- if (iommu_init_ga_log(iommu) ||
- iommu_ga_log_enable(iommu))
+ if (amd_iommu_gappi)
+ iommu_feature_enable(iommu, CONTROL_GAPPI_EN);
+ else if (iommu_init_ga_log(iommu) ||
+ iommu_ga_log_enable(iommu))
return;
iommu_feature_enable(iommu, CONTROL_GAM_EN);
@@ -3039,7 +3058,8 @@ static void enable_iommus_vapic(void)
}
amd_iommu_irq_ops.capability |= (1 << IRQ_POSTING_CAP);
- pr_info("Virtual APIC enabled\n");
+ pr_info("Virtual APIC enabled with %s\n",
+ amd_iommu_gappi ? "GAPPI" : "GALOG");
#endif
}
@@ -3782,6 +3802,12 @@ static int __init parse_amd_iommu_options(char *str)
} else if (strncmp(str, "v2_pgsizes_only", 15) == 0) {
pr_info("Restricting V1 page-sizes to 4KiB/2MiB/1GiB");
amd_iommu_pgsize_bitmap = AMD_IOMMU_PGSIZES_V2;
+ } else if (strncmp(str, "gappi", 5) == 0) {
+#ifdef CONFIG_IRQ_REMAP
+ amd_iommu_gappi = true;
+#else
+ pr_warn("amd_iommu=gappi ignored, IRQ remapping is not enabled\n");
+#endif
} else {
pr_notice("Unknown option - '%s'\n", str);
}
--
2.34.1