Re: [RFC PATCH 00/17] (The beginning of) a new i386 decoder

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

On 8/24/22 10:31, Paolo Bonzini wrote:
> It is only lightly tested but it can boot to iPXE and run some 64-bit
> coreutils just fine; Linux seems to trigger a bug in outsw/l/q emulation
> that I haven't checked yet, but still it's enough to show the result of
> a couple days of hacking.

Excellent.

> The generated code is mostly the same, though marginally worse in some
> cases because I privileged code simplicity.  For example, MOVSXD is not
> able to use MO_SL and falls back to MO_UL + sign extension.

I think this is ok.

We can improve things like this on a case-by-case basis.
For example, MOVSXD could gain a X86_SPECIAL_Signed flag,
to be passed on to gen_load().


> One notable
> difference is that the new decoder always sign-extends 8-bit immediates,
> so for example a "cmpb $e9, %dl" instruction will subtract $0xfff...fffe9
> from the temporary value.  This is the way Intel intended "Ib" immediates
> to work, and there's no difference between the two.

That is in fact an improvement.

> Anyay, porting these opcodes is really more of a validation for the
> whole concept and a test for the common decoder code; it's probably more
> efficient to focus on the SSE and VEX 2-byte and 3-byte opcodes as a path
> towards enabling AVX in QEMU, and keep the existing decoder for non-VEX,
> non-SSE opcodes.

Eh... I disagree.  I would really hate to retain the existing decoder.
This is already so much better...

> The series is available at https://gitlab.com/bonzini/qemu.git, branch i386.

Thanks.


r~