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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 13:37:51 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 08/06/2021 12:35, Richard Henderson wrote:
On 6/8/21 7:39 AM, Bruno Piazera Larsen wrote:
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.

According to tcg/ppc/tcg-target.c.inc, there are 8 argument registers for ppc hosts.  But now I see you didn't actually say on which host you observed the problem...  It's 6 argument registers for x86_64 host.

Oh, yes, sorry. I'm experiencing it in a POWER9 machine (ppc64le architecture). According to tcg this shouldn't be the issue, then, so idk if that's the real reason or not. All I know is that as soon as gcc can't optimize an argument away it happens (fprintf in radix64_xlate, using one of the mmuidx_* functions, defining those as macros).

I'll test it in my x86_64 machine and see if such a slowdown happens. It's not conclusive evidence, but the function is too complex for me to follow the disassembly if I can avoid it...

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.

Well, if you're going to change the xlate interface, you want to do that across all of them.  So, not call it radix_ctx_t.
I wouldn't change ppc_xlate's interface, I'd set up the struct in that function and call ppc_radix64_xlate using the struct

 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.

I don't think h/g should be in this struct.  I think h/g should use different struct instances, because they are different accesses.
Ok, makes sense

Bruno Piazera Larsen
Instituto de Pesquisas ELDORADO
Departamento Computação Embarcada
Analista de Software Trainee
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