Re: [Qemu-devel] [PULL 4/6] accel/tcg: Don't treat invalid TLB entries as needing recheck

[Richard Henderson]

On 07/13/2018 06:05 AM, Peter Maydell wrote:
>> -    if (unlikely(env->tlb_table[mmu_idx][index].addr_code & TLB_RECHECK)) {
>> +    if (unlikely((env->tlb_table[mmu_idx][index].addr_code &
>> +                  (TLB_RECHECK | TLB_INVALID_MASK)) == TLB_RECHECK)) {
>>          /*
>>           * This is a TLB_RECHECK access, where the MMU protection
>>           * covers a smaller range than a target page, and we must
> 
> Looking again at this code, I think that now we have the code to
> ensure that there's only ever one entry in the TLB/victim TLB for
> a given guest address...

Which probably wasn't the case the first time you wrote this, no?
Fixing that single entry condition was just a few weeks ago.


> The sequence
> 
>     if (unlikely(!tlb_hit(env->tlb_table[mmu_idx][index].addr_code, addr))) {
>         if (!VICTIM_TLB_HIT(addr_read, addr)) {
>             tlb_fill(ENV_GET_CPU(env), addr, 0, MMU_INST_FETCH, mmu_idx, 0);
>         }
>     }
> 
> should result in us always either (a) taking a guest exception and
> longjumping out of the tlb_fill(), or (b) ending up with the TLB
> containing an entry valid for an insn fetch, ie addr_code does not
> have TLB_INVALID_MASK set. So we could drop the check on TLB_INVALID_MASK
> here and instead have:
> 
>     assert(tlb_hit(env->tlb_table[mmu_idx][index].addr_code, addr));

Tuck that assert just after the tlb_fill, if you like.
I think it's unnecessary; we don't do that any of the
other places we use tlb_fill.


r~