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
r~
--
Bruno Piazera Larsen
Instituto de Pesquisas ELDORADO
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Departamento Computação Embarcada
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