[PATCH v2 2/2] riscv: T-Head: Test availability bit before enabling MAEE errata

[Christoph Müllner]

T-Head's MAEE mechanism (non-compatible equivalent of RVI's Svpbmt)
is currently assumed for all T-Head harts. However, QEMU recently
decided to drop acceptance of guests that write reserved bits in PTEs.
As MAEE uses reserved bits in PTEs and Linux applies the MAEE errata
for all T-Head harts, this broke the Linux startup on QEMU emulations
of the C906 emulation.

This patch attempts to address this issue by testing the MAEE bit
in the th.sxstatus CSR. This CSR is available in HW and can be
emulated in QEMU.

This patch also makes the MAEE probing mechanism reliable, because
a test for the right combination of mvendorid, marchid, and mimpid
is not sufficient to enable MAEE.

Signed-off-by: Christoph Müllner <christoph.muellner at vrull.eu>
---
 arch/riscv/errata/thead/errata.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
index 8c8a8a4b0421..dfa007039208 100644
--- a/arch/riscv/errata/thead/errata.c
+++ b/arch/riscv/errata/thead/errata.c
@@ -19,6 +19,9 @@
 #include <asm/patch.h>
 #include <asm/vendorid_list.h>
 
+#define CSR_TH_SXSTATUS		0x5c0
+#define SXSTATUS_MAEE		_AC(0x200000, UL)
+
 static bool errata_probe_maee(unsigned int stage,
 			      unsigned long arch_id, unsigned long impid)
 {
@@ -28,11 +31,14 @@ static bool errata_probe_maee(unsigned int stage,
 	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;
 
-	return false;
+	if (!(csr_read(CSR_TH_SXSTATUS) & SXSTATUS_MAEE))
+		return false;
+
+	return true;
 }
 
 /*
-- 
2.44.0