[RFC 2/9] cxl: Add preserve_config to pxb-cxl OSC method

mhonap
2026-04-27T18:14:31Z

Before this patch, pxb-cxl bridges had no _DSM method at all. When the
OS called _DSM on a CXL host bridge, ACPI returned an error and the OS
defaulted to reassigning resources across suspend/resume. On machines
where firmware pre-commits the HDM decoder, that reassignment breaks the
DPA mapping.

Wire preserve_config through GPEXConfig into build_cxl_osc_method() so
pxb-cxl host bridges get a _DSM method that signals the OS to keep
resource assignments stable when needed. The _DSM function 5 (preserve
firmware PCI configuration) is the mechanism used to convey this.

build_pci_host_bridge_dsm_method() is promoted from static to exported
so cxl.c can call it without duplicating the AML.

The x86 build_cxl_osc_method() call site passes false since x86 does
not use firmware-committed HDM decoders.

build_cxl_osc_method is renamed to acpi_dsdt_add_cxl_host_bridge_methods
The function now appends both the CXL _OSC method and the _DSM method,
so its old name is misleading. Renamed it to match the pxb-pcie analogue
acpi_dsdt_add_host_bridge_methods(), making the two root bridge code
paths symmetric. No AML change.

Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
Signed-off-by: Manish Honap <mhonap@nvidia.com>
---
hw/acpi/cxl-stub.c | 2 +-
hw/acpi/cxl.c | 4 +++-
hw/i386/acpi-build.c | 2 +-
hw/pci-host/gpex-acpi.c | 5 +++--
include/hw/acpi/cxl.h | 2 +-
include/hw/pci-host/gpex.h | 1 +
6 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/hw/acpi/cxl-stub.c b/hw/acpi/cxl-stub.c
index 15bc21076b..d7c6731975 100644
--- a/hw/acpi/cxl-stub.c
+++ b/hw/acpi/cxl-stub.c
@@ -6,7 +6,7 @@
#include "hw/acpi/aml-build.h"
#include "hw/acpi/cxl.h"

-void build_cxl_osc_method(Aml *dev)
+void acpi_dsdt_add_cxl_host_bridge_methods(Aml *dev, bool preserve_config)
{
g_assert_not_reached();
}
diff --git a/hw/acpi/cxl.c b/hw/acpi/cxl.c
index f92f7fa3d5..b32740a3e3 100644
--- a/hw/acpi/cxl.c
+++ b/hw/acpi/cxl.c
@@ -23,6 +23,7 @@
#include "hw/pci/pci_host.h"
#include "hw/cxl/cxl.h"
#include "hw/cxl/cxl_host.h"
+#include "hw/pci-host/gpex.h"
#include "hw/mem/memory-device.h"
#include "hw/acpi/acpi.h"
#include "hw/acpi/aml-build.h"
@@ -320,11 +321,12 @@ static Aml *__build_cxl_osc_method(void)
return method;
}

-void build_cxl_osc_method(Aml *dev)
+void acpi_dsdt_add_cxl_host_bridge_methods(Aml *dev, bool preserve_config)
{
aml_append(dev, aml_name_decl("SUPP", aml_int(0)));
aml_append(dev, aml_name_decl("CTRL", aml_int(0)));
aml_append(dev, aml_name_decl("SUPC", aml_int(0)));
aml_append(dev, aml_name_decl("CTRC", aml_int(0)));
aml_append(dev, __build_cxl_osc_method());
+ aml_append(dev, build_pci_host_bridge_dsm_method(preserve_config));
}
diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index f622b91b76..f66ec8ed24 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -1013,7 +1013,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
aml_append(aml_pkg, aml_eisaid("PNP0A08"));
aml_append(aml_pkg, aml_eisaid("PNP0A03"));
aml_append(dev, aml_name_decl("_CID", aml_pkg));
- build_cxl_osc_method(dev);
+ acpi_dsdt_add_cxl_host_bridge_methods(dev, false);
} else if (pci_bus_is_express(bus)) {
aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0A08")));
aml_append(dev, aml_name_decl("_CID", aml_eisaid("PNP0A03")));
diff --git a/hw/pci-host/gpex-acpi.c b/hw/pci-host/gpex-acpi.c
index 7de57bbc46..247bd78152 100644
--- a/hw/pci-host/gpex-acpi.c
+++ b/hw/pci-host/gpex-acpi.c
@@ -52,7 +52,7 @@ static void acpi_dsdt_add_pci_route_table(Aml *dev, uint32_t irq,
}
}

-static Aml *build_pci_host_bridge_dsm_method(bool preserve_config)
+Aml *build_pci_host_bridge_dsm_method(bool preserve_config)
{
Aml *method = aml_method("_DSM", 4, AML_NOTSERIALIZED);
Aml *UUID, *ifctx, *ifctx1, *buf;
@@ -204,7 +204,8 @@ void acpi_dsdt_add_gpex(Aml *scope, struct GPEXConfig *cfg)
aml_append(dev, aml_name_decl("_CRS", crs));

if (is_cxl) {
- build_cxl_osc_method(dev);
+ acpi_dsdt_add_cxl_host_bridge_methods(dev,
+ cfg->preserve_config);
} else {
/* pxb bridges do not have ACPI PCI Hot-plug enabled */
acpi_dsdt_add_host_bridge_methods(dev, true,
diff --git a/include/hw/acpi/cxl.h b/include/hw/acpi/cxl.h
index 8f22c71530..6fe6c9c58d 100644
--- a/include/hw/acpi/cxl.h
+++ b/include/hw/acpi/cxl.h
@@ -24,7 +24,7 @@
void cxl_build_cedt(GArray *table_offsets, GArray *table_data,
BIOSLinker *linker, const char *oem_id,
const char *oem_table_id, CXLState *cxl_state);
-void build_cxl_osc_method(Aml *dev);
+void acpi_dsdt_add_cxl_host_bridge_methods(Aml *dev, bool preserve_config);
void build_cxl_dsm_method(Aml *dev);

#endif
diff --git a/include/hw/pci-host/gpex.h b/include/hw/pci-host/gpex.h
index a7c2e2edf3..e5c2ebef78 100644
--- a/include/hw/pci-host/gpex.h
+++ b/include/hw/pci-host/gpex.h
@@ -73,6 +73,7 @@ struct GPEXHost {
int gpex_set_irq_num(GPEXHost *s, int index, int gsi);

void acpi_dsdt_add_gpex(Aml *scope, struct GPEXConfig *cfg);
+Aml *build_pci_host_bridge_dsm_method(bool preserve_config);
void acpi_dsdt_add_gpex_host(Aml *scope, uint32_t irq);

#define PCI_HOST_PIO_BASE "x-pio-base"
--
2.25.1