Merge patch series "RISC-V: Test th.sxstatus.MAEE bit before enabling MAEE"

Christoph Müllner <christoph.muellner@vrull.eu> says:

Currently, the Linux kernel suffers from a boot regression when running
on the c906 QEMU emulation. Details have been reported here by Björn Töpel:
  https://lists.gnu.org/archive/html/qemu-devel/2024-01/msg04766.html

The main issue is, that Linux enables XTheadMae for CPUs that have a T-Head
mvendorid but QEMU maintainers don't want to emulate a CPU that uses
reserved bits in PTEs. See also the following discussion for more
context:
  https://lists.gnu.org/archive/html/qemu-devel/2024-02/msg00775.html

This series renames "T-Head PBMT" to "MAE"/"XTheadMae" and only enables
it if the th.sxstatus.MAEE bit is set.

The th.sxstatus CSR is documented here:
  https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadsxstatus.adoc

XTheadMae is documented here:
  https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadmae.adoc

The QEMU patch to emulate th.sxstatus with the MAEE bit not set is here:
  https://lore.kernel.org/all/20240329120427.684677-1-christoph.muellner@vrull.eu/

After applying the referenced QEMU patch, this patchset allows to
successfully boot a C906 QEMU system emulation ("-cpu thead-c906").

* b4-shazam-lts:
  riscv: T-Head: Test availability bit before enabling MAE errata
  riscv: thead: Rename T-Head PBMT to MAE

Link: https://lore.kernel.org/r/20240407213236.2121592-1-christoph.muellner@vrull.eu
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

3 files changed, 29 insertions, 23 deletions

diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
index 910ba8837add..2acc7d876e1f 100644
--- a/arch/riscv/Kconfig.errata
+++ b/arch/riscv/Kconfig.errata
@@ -82,14 +82,14 @@ config ERRATA_THEAD
 
 	  Otherwise, please say "N" here to avoid unnecessary overhead.
 
-config ERRATA_THEAD_PBMT
-	bool "Apply T-Head memory type errata"
+config ERRATA_THEAD_MAE
+	bool "Apply T-Head's memory attribute extension (XTheadMae) errata"
 	depends on ERRATA_THEAD && 64BIT && MMU
 	select RISCV_ALTERNATIVE_EARLY
 	default y
 	help
-	  This will apply the memory type errata to handle the non-standard
-	  memory type bits in page-table-entries on T-Head SoCs.
+	  This will apply the memory attribute extension errata to handle the
+	  non-standard PTE utilization on T-Head SoCs (XTheadMae).
 
 	  If you don't know what to do here, say "Y".
 
diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
index b1c410bbc1ae..bf6a0a6318ee 100644
--- a/arch/riscv/errata/thead/errata.c
+++ b/arch/riscv/errata/thead/errata.c
@@ -19,20 +19,26 @@
 #include <asm/patch.h>
 #include <asm/vendorid_list.h>
 
-static bool errata_probe_pbmt(unsigned int stage,
-			      unsigned long arch_id, unsigned long impid)
+#define CSR_TH_SXSTATUS		0x5c0
+#define SXSTATUS_MAEE		_AC(0x200000, UL)
+
+static bool errata_probe_mae(unsigned int stage,
+			     unsigned long arch_id, unsigned long impid)
 {
-	if (!IS_ENABLED(CONFIG_ERRATA_THEAD_PBMT))
+	if (!IS_ENABLED(CONFIG_ERRATA_THEAD_MAE))
 		return false;
 
 	if (arch_id != 0 || impid != 0)
 		return false;
 
-	if (stage == RISCV_ALTERNATIVES_EARLY_BOOT ||
-	    stage == RISCV_ALTERNATIVES_MODULE)
-		return true;
+	if (stage != RISCV_ALTERNATIVES_EARLY_BOOT &&
+	    stage != RISCV_ALTERNATIVES_MODULE)
+		return false;
+
+	if (!(csr_read(CSR_TH_SXSTATUS) & SXSTATUS_MAEE))
+		return false;
 
-	return false;
+	return true;
 }
 
 /*
@@ -140,8 +146,8 @@ static u32 thead_errata_probe(unsigned int stage,
 {
 	u32 cpu_req_errata = 0;
 
-	if (errata_probe_pbmt(stage, archid, impid))
-		cpu_req_errata |= BIT(ERRATA_THEAD_PBMT);
+	if (errata_probe_mae(stage, archid, impid))
+		cpu_req_errata |= BIT(ERRATA_THEAD_MAE);
 
 	errata_probe_cmo(stage, archid, impid);
 
diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
index 1f2dbfb8a8bf..efd851e1b483 100644
--- a/arch/riscv/include/asm/errata_list.h
+++ b/arch/riscv/include/asm/errata_list.h
@@ -23,7 +23,7 @@
 #endif
 
 #ifdef CONFIG_ERRATA_THEAD
-#define	ERRATA_THEAD_PBMT 0
+#define	ERRATA_THEAD_MAE 0
 #define	ERRATA_THEAD_PMU 1
 #define	ERRATA_THEAD_NUMBER 2
 #endif
@@ -53,20 +53,20 @@ asm(ALTERNATIVE("sfence.vma %0", "sfence.vma", SIFIVE_VENDOR_ID,	\
  * in the default case.
  */
 #define ALT_SVPBMT_SHIFT 61
-#define ALT_THEAD_PBMT_SHIFT 59
+#define ALT_THEAD_MAE_SHIFT 59
 #define ALT_SVPBMT(_val, prot)						\
 asm(ALTERNATIVE_2("li %0, 0\t\nnop",					\
 		  "li %0, %1\t\nslli %0,%0,%3", 0,			\
 			RISCV_ISA_EXT_SVPBMT, CONFIG_RISCV_ISA_SVPBMT,	\
 		  "li %0, %2\t\nslli %0,%0,%4", THEAD_VENDOR_ID,	\
-			ERRATA_THEAD_PBMT, CONFIG_ERRATA_THEAD_PBMT)	\
+			ERRATA_THEAD_MAE, CONFIG_ERRATA_THEAD_MAE)	\
 		: "=r"(_val)						\
 		: "I"(prot##_SVPBMT >> ALT_SVPBMT_SHIFT),		\
-		  "I"(prot##_THEAD >> ALT_THEAD_PBMT_SHIFT),		\
+		  "I"(prot##_THEAD >> ALT_THEAD_MAE_SHIFT),		\
 		  "I"(ALT_SVPBMT_SHIFT),				\
-		  "I"(ALT_THEAD_PBMT_SHIFT))
+		  "I"(ALT_THEAD_MAE_SHIFT))
 
-#ifdef CONFIG_ERRATA_THEAD_PBMT
+#ifdef CONFIG_ERRATA_THEAD_MAE
 /*
  * IO/NOCACHE memory types are handled together with svpbmt,
  * so on T-Head chips, check if no other memory type is set,
@@ -83,11 +83,11 @@ asm volatile(ALTERNATIVE(						\
 	"slli    t3, t3, %3\n\t"					\
 	"or      %0, %0, t3\n\t"					\
 	"2:",  THEAD_VENDOR_ID,						\
-		ERRATA_THEAD_PBMT, CONFIG_ERRATA_THEAD_PBMT)		\
+		ERRATA_THEAD_MAE, CONFIG_ERRATA_THEAD_MAE)		\
 	: "+r"(_val)							\
-	: "I"(_PAGE_MTMASK_THEAD >> ALT_THEAD_PBMT_SHIFT),		\
-	  "I"(_PAGE_PMA_THEAD >> ALT_THEAD_PBMT_SHIFT),			\
-	  "I"(ALT_THEAD_PBMT_SHIFT)					\
+	: "I"(_PAGE_MTMASK_THEAD >> ALT_THEAD_MAE_SHIFT),		\
+	  "I"(_PAGE_PMA_THEAD >> ALT_THEAD_MAE_SHIFT),			\
+	  "I"(ALT_THEAD_MAE_SHIFT)					\
 	: "t3")
 #else
 #define ALT_THEAD_PMA(_val)