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

From: Bruno Piazera Larsen
Subject: Re: [RFC PATCH] target/ppc: fix address translation bug for hash table mmus
Date: Tue, 8 Jun 2021 11:39:24 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 07/06/2021 18:06, Richard Henderson wrote:
On 6/7/21 12:29 PM, Bruno Piazera Larsen wrote:
I just tried sending mmu_idx all the way down, but I ran into a very weird bug of gcc. If we have to add one more parameter that GCC can't just optimize away we get at least a slow down of 5x for the first test of check-acceptance (could be more, but the test times out after 900 seconds, so I'm not sure).

That's odd.  We already have more arguments than the number of argument registers...  A 5x slowdown is distinctly odd.
I did some more digging and the problem is not with ppc_radix64_check_prot, the problem is ppc_radix64_xlate, which currently has 7 arguments and we're increasing to 8. 7 feels like the correct number, but I couldn't find docs supporting it, so I could be wrong.

One way that I managed to get around that is saving the current MSR, setting it to 5, and restoring after the xlate call. The code ended up something like:

int new_idx = (5<<HFLAGS_IMMU_IDX) | (5<<HFLAGS_DMMU_IDX);
int clr = (7<<HFLAGS_IMMU_IDX) | (7<<HFLAGS_DMMU_IDX);
int old_idx = env->msr & clr;
clr = ~clr;
/* set new msr so we don't need to send the mmu_idx */
env->msr = (env->msr & clr) | new_idx;
ret = ppc_radix64_partition_scoped_xlate(...);
/* restore old mmu_idx */
env->msr = (env->msr & clr) | old_idx;

No, this is silly.

We need to do one of two things:
  - make sure everything is inlined,
  - reduce the number of arguments.

We're currently passing in 9 arguments, which really is too many already.  We should be using something akin to mmu_ctx_t, but probably specific to radix64 without the random stuff collected for random other mmu models.

That means we'd have to define radix_ctx_t (or however we call it) in radix64.h, setup the struct on ppc_xlate, then pass it to ppc_radix64_xlate.

From looking at the code, it seems the most useful bits to put in the struct are: eaddr, g_addr, h_addr, {h,g}_prot, {g,h}_page_size, mmu_idx and guest_visible. They all seem reasonable to me, but I might be missing something again.

Another question: I know hash mmus don't have this problem, but since ppc_jumbo_xlate also uses mmu_idx, should we make those xlate user mmu_idxs as well? I tested and it doesn't make a time difference.
Bruno Piazera Larsen
Instituto de Pesquisas ELDORADO
Departamento Computação Embarcada
Analista de Software Trainee
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