Re: [PATCH] x86emul: drop host_and_vcpu_must_have()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/07/2026 10:46 am, Jan Beulich wrote:
> With CPU policy dependency logic now properly in place and working, and
> with us never enabling ISA bits in guest policies which the underlying
> hardware doesn't support, the dual checks aren't needed anymore. In fact
> in Misra terms they are dead code.

This absolutely cannot be called dead code, MISRA or otherwise.

We have a bunch of logic which is checking for A && B.  A and B are
distinct objects in memory, with A always being evaluated, B being
conditionally evaluated, and both having input into the final calculation.

We believe that in all cases, A has the same value as B, and therefore
the logic can be simplified to just B, but that is a logical leap, not a
transformation that any tooling can make.

>
> In turn, a number of cpu_has_* can then also go away, when their only user
> was the emulator code.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> In my build (with all the other pending work in place) this is a savings
> of about 2k of generated code.
>
> A related question is in how far vcpu_has_*() are useful to retain. We
> could as well access the policy directly, which would even be more
> efficient when a "cp" local variable (or function parameter) is in scope.
> Then x86_emulate()'s instance could also have its __maybe_unused dropped.

You'd need the test harness able to use a struct cpu_policy in order to
drop vcpu_has_*().  It's doable, but isn't a small change.

> With the cpu_has_* being dropped I came to wonder why we have e.g.
> cpu_has_lkgs. This particular one was added together with cpu_has_fred,
> yet imo we better wouldn't add such #define-s when they're not going to be
> used.

Older versions of the FRED series checked for cpu_has_fred &&
cpu_has_lkgs, but I dropped the latter on discovering that Xen didn't
need an LKGS instruction at all.

I think I will need in order to get the guest policies working
correctly, so please don't drop it yet.

~Andrew