Re: [Xen-devel] [PATCH v2 1/2] x86emul: make _PRE_EFLAGS() tolerate first argument being 32-bit

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/01/17 10:22, Jan Beulich wrote:
> While this may appear to introduce a truncation issue, the high 32 bits
> get zapped already anyway (early in _PRE_EFLAGS() as well as in
> _POST_EFLAGS()). Once a subsequent patch switches to use proper 32-bit
> EFLAGS operands, we'll in fact end up with more correct code, as that
> zeroing of the upper halves will then go away.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> As this adds an instruction, the question is whether it would be worth
> forking _PRE_EFLAGS() into two flavors: One dealing with _sav in a
> register (allowing several instructions to be dropped) and another
> dealing with it being on the stack (in which case the logic needs to
> remain as is, since between the first PUSH and the last POP we mustn't
> access variables possibly living on the stack).

Looking at the code, why does so much of this need to be written in
ASM?  Most looks like it could be moved into C.

All that is needed in ASM is something like:

push %[flags_before]
popf
... op ...
pushf
pop %[flags_after]

And the actual masking calculations can be done in C.

I'd suggest putting this patch in as-is to unblock the register
renaming, and defer any potential improvements to separate work.

~Andrew

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