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Re: [PATCH] target/riscv/pmp: fix non-translated page size address check

From: Leon Schuermann
Subject: Re: [PATCH] target/riscv/pmp: fix non-translated page size address checks w/ MPU
Date: Wed, 19 Oct 2022 17:29:04 -0400

Alistair Francis <alistair23@gmail.com> writes:
>> @@ -310,10 +311,17 @@ bool pmp_hart_has_privs(CPURISCVState *env, 
>> target_ulong addr,
>>      }
>>      if (size == 0) {
>> -        if (riscv_feature(env, RISCV_FEATURE_MMU)) {
>> +        if (riscv_cpu_mxl(env) == MXL_RV32) {
>> +            satp_mode = SATP32_MODE;
>> +        } else {
>> +            satp_mode = SATP64_MODE;
>> +        }
>> +
>> +        if (riscv_feature(env, RISCV_FEATURE_MMU)
>> +            && get_field(env->satp, satp_mode)) {
>>              /*
>> -             * If size is unknown (0), assume that all bytes
>> -             * from addr to the end of the page will be accessed.
>> +             * If size is unknown (0) and virtual memory is enabled, assume 
>> that
>> +             * all bytes from addr to the end of the page will be accessed.
>>               */
>>              pmp_size = -(addr | TARGET_PAGE_MASK);
> I'm not sure if we need this at all.
> This function is only called from get_physical_address_pmp() which
> then calculates the maximum size using pmp_is_range_in_tlb().

I'm by no means an expert on QEMU and the TCG, so I've spun up GDB to
trace down why exactly this function is called with a `size = 0`
argument. It seems that there are, generally, two code paths to this
function for instruction fetching:

1. From `get_page_addr_code`: this will invoke `tlb_fill` with
   `size = 0` to check whether an entire page is accessible and can be
   translated given the current MMU / PMP configuration. In my
   particular example, it may rightfully fail then. `get_page_addr_code`
   can handle this and will subsequently cause an MMU protection check
   to be run for each instruction translated.

2. From `riscv_tr_translate_insn` through `cpu_lduw_code`, which will
   execute `tlb_fill` with `size = 2` (to try and decode a compressed
   instruction), assuming that the above check failed.

So far, so good. In this context, it actually makes sense for
`pmp_hart_has_privs` to interpret `size = 0` to mean whether the entire
page is allowed to be accessed.

> I suspect that we could just use sizeof(target_ulong) as the fallback
> for every time size == 0. Then pmp_is_range_in_tlb() will set the
> tlb_size to the maximum possible size of the PMP region.

Given the above, I don't think that this is correct either. The PMP
check would pass even for non-page sized regions, but the entire page
would be accessible through the TCG's TLB, as a consequence of

In the current implementation, `get_page_addr_code_hostp` calls
`tlb_fill`, which in turn invokes the RISC-V TCG op `tlb_fill` with the
parameter `probe = false`. This in turn raises a PMP exception in the
CPU, whereas `get_page_addr_code` would seem to expect this a failing
`tlb_fill` to be side-effect free, such that the MMU protection checks
can be re-run per instruction in the TCG code generation phase.

I think that this is sufficient evidence to conclude that my initial
patch is actually incorrect, however I am unsure as to how this issue
can be solved proper. One approach which seems to work is to change
`get_page_addr_code_hostp` to use a non-faulting page-table read

@@ -1510,11 +1510,15 @@ tb_page_addr_t get_page_addr_code_hostp(CPUArchState 
*env, target_ulong addr,
     uintptr_t mmu_idx = cpu_mmu_index(env, true);
     uintptr_t index = tlb_index(env, mmu_idx, addr);
     CPUTLBEntry *entry = tlb_entry(env, mmu_idx, addr);
+    CPUState *cs = env_cpu(env);
+    CPUClass *cc = CPU_GET_CLASS(cs);
     void *p;
     if (unlikely(!tlb_hit(entry->addr_code, addr))) {
         if (!VICTIM_TLB_HIT(addr_code, addr)) {
-            tlb_fill(env_cpu(env), addr, 0, MMU_INST_FETCH, mmu_idx, 0);
+            // Nonfaulting page-table read:
+            cc->tcg_ops->tlb_fill(cs, addr, 0, MMU_INST_FETCH, mmu_idx, true,
+                                  0);
             index = tlb_index(env, mmu_idx, addr);
             entry = tlb_entry(env, mmu_idx, addr);

However, given this touches the generic TCG implementation, I cannot
judge whether this is correct or has any unintended side effects for
other targets. If this is correct, I'd be happy to send a proper patch.


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