[PATCH v2 2/2] target/riscv/cpu_helper.c: Fix mxr bit behavior

[Ivan Klokov <ivan.klokov@syntacore.com>]

According to RISCV Specification sect 9.5 on two stage translation when
V=1 the vsstatus(mstatus in QEMU's terms) field MXR, which makes
execute-only pages readable, only overrides VS-stage page protection.
Setting MXR at HS-level(mstatus_hs), however, overrides both VS-stage
and G-stage execute-only permissions.

The hypervisor extension changes the behavior of MXR\MPV\MPRV bits.
Due to RISCV Specification sect. 9.4.1 when MPRV=1, explicit memory
accesses are translated and protected, and endianness is applied, as
though the current virtualization mode were set to MPV and the current
nominal privilege mode were set to MPP. vsstatus.MXR makes readable
those pages marked executable at the VS translation stage.

Fixes: 36a18664ba ("target/riscv: Implement second stage MMU")

Signed-off-by: Ivan Klokov <ivan.klokov@syntacore.com>
---
 target/riscv/cpu_helper.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 9ff0952e46..e7e23b34f4 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -1032,13 +1032,29 @@ restart:
         prot |= PAGE_WRITE;
     }
     if (pte & PTE_X) {
-        bool mxr;
+        bool mxr = false;
 
-        if (first_stage == true) {
+        /*
+         * Use mstatus for first stage or for the second stage without
+         * virt_enabled (MPRV+MPV)
+         */
+        if (first_stage || !env->virt_enabled) {
             mxr = get_field(env->mstatus, MSTATUS_MXR);
-        } else {
-            mxr = get_field(env->vsstatus, MSTATUS_MXR);
         }
+
+        /* MPRV+MPV case, check VSSTATUS */
+        if (first_stage && two_stage && !env->virt_enabled) {
+            mxr |= get_field(env->vsstatus, MSTATUS_MXR);
+        }
+
+        /*
+         * Setting MXR at HS-level overrides both VS-stage and G-stage
+         * execute-only permissions
+         */
+        if (env->virt_enabled) {
+            mxr |= get_field(env->mstatus_hs, MSTATUS_MXR);
+        }
+
         if (mxr) {
             prot |= PAGE_READ;
         }
-- 
2.34.1