riscv: T-Head: Test availability bit before enabling MAE errata

T-Head's memory attribute extension (XTheadMae) (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 XTheadMae uses reserved bits in PTEs and Linux applies the MAE 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 MAE-enable bit
in the th.sxstatus CSR. This CSR is available in HW and can be
emulated in QEMU.

This patch also makes the XTheadMae probing mechanism reliable, because
a test for the right combination of mvendorid, marchid, and mimpid
is not sufficient to enable MAE.
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Link: https://lore.kernel.org/r/20240407213236.2121592-3-christoph.muellner@vrull.euSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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_mae(unsigned int stage,
 			     unsigned long arch_id, unsigned long impid)
 {
@@ -28,11 +31,14 @@ static bool errata_probe_mae(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;
 }
 
 /*