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Re: [PATCH v1 1/1] target/riscv: Don't set write permissions on dirty PT

From: Palmer Dabbelt
Subject: Re: [PATCH v1 1/1] target/riscv: Don't set write permissions on dirty PTEs
Date: Wed, 18 Mar 2020 21:52:43 -0700 (PDT)

On Tue, 03 Mar 2020 17:16:59 PST (-0800), Alistair Francis wrote:
The RISC-V spec specifies that when a write happens and the D bit is
clear the implementation will set the bit in the PTE. It does not
describe that the PTE being dirty means that we should provide write
access. This patch removes the write access granted to pages when the
dirty bit is set.

Following the prot variable we can see that it affects all of these

Looking at the cputlb code (tlb_set_page_with_attrs() and
address_space_translate_for_iotlb()) it looks like the main affect of
setting write permissions is that the page can be marked as TLB_NOTDIRTY.

I don't see any other impacts (related to the dirty bit) for giving a
page write permissions.

Setting write permission on dirty PTEs results in userspace inside a
Hypervisor guest (VU) becoming corrupted. This appears to be because it
ends up with write permission in the second stage translation in cases
where we aren't doing a store.

Signed-off-by: Alistair Francis <address@hidden>
Reviewed-by: Bin Meng <address@hidden>
 target/riscv/cpu_helper.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 5ea5d133aa..cc9f20b471 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -572,10 +572,8 @@ restart:
             if ((pte & PTE_X)) {
                 *prot |= PAGE_EXEC;
-            /* add write permission on stores or if the page is already dirty,
-               so that we TLB miss on later writes to update the dirty bit */
-            if ((pte & PTE_W) &&
-                    (access_type == MMU_DATA_STORE || (pte & PTE_D))) {
+            /* add write permission on stores */
+            if ((pte & PTE_W) && (access_type == MMU_DATA_STORE)) {
                 *prot |= PAGE_WRITE;
             return TRANSLATE_SUCCESS;

I remember having seen this patch before and having some objections, but I feel
like I mistakenly had this backwards before or something because it makes sense


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