target/riscv: pmp: Fixup TLB size calculation

Since commit 4047368 "accel/tcg: Introduce tlb_set_page_full" we
have been seeing this assert

    ../accel/tcg/cputlb.c:1294: tlb_set_page_with_attrs: Assertion `is_power_of_2(size)' failed.

When running Tock on the OpenTitan machine.

The issue is that pmp_get_tlb_size() would return a TLB size that wasn't
a power of 2. The size was also smaller then TARGET_PAGE_SIZE.

This patch ensures that any TLB size less then TARGET_PAGE_SIZE is
rounded down to 1 to ensure it's a valid size.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei<zhiwei_liu@linux.alibaba.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20221012011449.506928-1-alistair.francis@opensource.wdc.com
Message-Id: <20221012011449.506928-1-alistair.francis@opensource.wdc.com>

target/riscv/pmp.c

@@ -628,6 +628,18 @@ bool pmp_is_range_in_tlb(CPURISCVState *env, hwaddr tlb_sa,
     }
 
     if (*tlb_size != 0) {
+        /*
+         * At this point we have a tlb_size that is the smallest possible size
+         * That fits within a TARGET_PAGE_SIZE and the PMP region.
+         *
+         * If the size is less then TARGET_PAGE_SIZE we drop the size to 1.
+         * This means the result isn't cached in the TLB and is only used for
+         * a single translation.
+         */
+        if (*tlb_size < TARGET_PAGE_SIZE) {
+            *tlb_size = 1;
+        }
+
         return true;
     }