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.eu
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
index 6e7ee1f16bee3e3008b335bfd7c6234ad2ac51bd..bf6a0a6318ee307085c3cc04c4056a0b63a000e9 100644 (file)
--- 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_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;
 }
 
 /*