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Re: [PATCH] target/riscv: Smepmp: Skip applying default rules when addre

From: Himanshu Chauhan
Subject: Re: [PATCH] target/riscv: Smepmp: Skip applying default rules when address matches
Date: Mon, 13 Feb 2023 10:51:37 +0530
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1

On 13/02/23 09:52, LIU Zhiwei wrote:

On 2023/2/9 13:52, Himanshu Chauhan wrote:
When MSECCFG.MML is set, after checking the address range in PMP if the
asked permissions are not same as programmed in PMP, the default
permissions are applied. This should only be the case when there
is no matching address is found.

This patch skips applying default rules when matching address range
is found. It returns the index of the match PMP entry.

fixes: 824cac681c3 (target/riscv: Fix PMP propagation for tlb)

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
  target/riscv/pmp.c | 9 ++++++---
  1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/target/riscv/pmp.c b/target/riscv/pmp.c
index d85ad07caa..0dfdb35828 100644
--- a/target/riscv/pmp.c
+++ b/target/riscv/pmp.c
@@ -446,9 +446,12 @@ int pmp_hart_has_privs(CPURISCVState *env, target_ulong addr,
  -            if ((privs & *allowed_privs) == privs) {
-                ret = i;
-            }
+            /*
+             * If matching address range was found, the protection bits
+             * defined with PMP must be used. We shouldn't fallback on
+             * finding default privileges.
+             */
+            ret = i;

Notice the return value is the matching rule index, which includes

1) the address range is matching.

2) the permission of the PMP rule and the memory access type are matching.

So we can't simply remove the second check.  I think the right fix is:

           if ((privs & *allowed_privs) == privs) {
                ret = i;
 -         }
 +         } else {
 +        ret = -2;
 +         }

The -2 return value avoids finding the default privileges. And it implies no matching rule is found.


Hi Zhiwei,

In case the address range is matched and MSECCFG.MML is set, the permission in *allowed_privs* are binding. So if the index matching is returned, the binding permissions are applied by the caller function.

Which case does my patch break?

- Himanshu


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