Re: Emulate if x86-64 on x86-64

[Jakob Bohm]

On 2024-08-13 18:43, Peter Maydell wrote:
> On Tue, 13 Aug 2024 at 16:54, Jakob Bohm via <qemu-discuss@nongnu.org> wrote:
> > Another approach that might be worth examining would be to build the TCG
> > snippet collection for different host CPU generations (such as x86-64
> > with only SSE2, x86-64 with SSE3 and x86-64 with AVX512), thereby
> > providing fast emulation of the latest on the new on the new and medium
> > CPU generations .  This would imply some table selection logic in the
> > TCG initialization based on actual host and selected guest CPUs. This
> > idea is mostly for Qemu binaries that will run on different hosts .
>
> What "TCG snippet collection" ? QEMU generates code from IR
> instruction-at-a-time, it doesn't do assembly of prebuilt
> larger bits of code.

I was referring to the part of the old Qemu build process that compiles
a bunch of TCG equivalents, at least when the host CPU has no hardcoded
translations for the guest CPU.

> We do have some support for runtime detection of host
> CPU features, so we can generate better code if some
> things are available; notably we will check for and
> use AVX1, AVX2, AVX512 if available.

This seems to leave out the cases of SSE3 orig on x86-64
and SSE on 80386SX .


Enjoy

Jakob
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