Re: [RFC PATCH] target/ppc: fix address translation bug for hash table mmus

[Richard Henderson <richard.henderson@linaro.org>]

On 6/2/21 12:58 PM, Bruno Piazera Larsen wrote:
>> For the use from ppc_cpu_get_phys_page_debug, you'd pass in 
>> cpu_mmu_index(env, false).
> 
> ppc_cpu_get_phys_page_debug has 2 calls to ppc_xlate, one using the data MMU, 
> the other using the instruction MMU. I'm guessing I should pass both, right?

Yes.

> But here we have another bit that confuses me: cpu_mmu_index returns 0 if in 
> user mode, or uses the information stored in env to get it, so I don't see how 
> that would be different from getting directly. Unless the point is to have 
> ppc_*_xlate be generic and pc_*_debug knows the info in env is correct. Is that it?

The issue is that

(1) ppc_*_xlate should perform the lookup requested, and mmu_idx
     does not *necessarily* correspond to the current contents of
     env->msr et al.  See (2).

(2) There is a secondary call to ppc_radix64_partition_scoped_xlate
     for which the second stage page table should be read
     with hypervisor permissions, and not the permissions of the
     original memory access.

     Note that ppc_radix64_check_prot checks msr_pr directly.

     Thus the second stage lookup should use mmu_idx = 5
     (HV kernel virtual mode).  If I understand things correctly...

> 
>>
>>
>>> +    const short HV = 1, IR = 2, DR = 3;
>>> +    bool MSR[3];
>>> +    MSR[HV] = dmmu_idx & 2,
>>> +    MSR[IR] = immu_idx & 4,
>>> +    MSR[DR] = dmmu_idx & 4;
>>
>> There's no point in the array.  Just use three different scalars (real_mode, 
>> hv, and pr (note that pr is the major portion of the bug as reported)). 
>> Additionally, you'll not be distinguishing immu_idx and dmmu_idx, but using 
>> the single idx that's given.
> 
> Ah, yeah, that's the "more complex than necessary, but it was easy for me to 
> read" part. Scalars are a good solution. In this function in specific, PR 
> doesn't actually show up anywhere, so I would actually only need 2. Anyway, 
> will start working on this.

Oh, I'll note that your constants above are wrong.  I think that you should 
have some common routines in (mmu-)internal.h:

/*
  * These correspond to the mmu_idx values computed in
  * hreg_compute_hflags_value.  See the tables therein.
  */
static inline bool mmuidx_pr(int idx) { return idx & 1; }
static inline bool mmuidx_real(int idx) { return idx & 2; }
static inline bool mmuidx_hv(int idx) { return idx & 4; }

because you'll want to use these past mmu-radix64.c.

Then you also have a single place to adjust if the mmu_idx are reordered at a 
later date.


r~