2014-09-17 17:22+0200, Borislav Petkov:
> On Wed, Sep 17, 2014 at 05:04:33PM +0200, Radim Krčmář wrote:
> > which would result in a similar if-else hack
> >
> > if (family > X)
> > ebx.split.max_monitor_line_size_after_family_X = 0
> > else
> > ebx.split.max_monitor_line_size = 0
> >
> > other options are
> > ebx.split.after_family_X.max_monitor_line_size
> > or even
> > ebx.split.max_monitor_line_size.after_family_X
>
> And how is that better than simply doing
>
> cpuid = cpuid_ebx(5);
>
> if (family > X)
> max_monitor_line_size = cpuid & MASK_FAM_X;
> else
> max_monitor_line_size = cpuid & MASK_BEFORE_FAM_X;
>
> ?
>
> With proper variable naming all is perfectly clear, readable
> and simple. You don't need to open even the CPUID manual - the
> variable tells you you're getting the max monitor line size -
> "ebx.split.max_monitor_line_size_after_family_X" needs me to parse it
> with my eyes first.
I think you proposed to use magic constant in place of of MASK_FAM_X, so
the code above is
if (family > X)
max_monitor_line_size = cpuid & 0x1ffff;
else
max_monitor_line_size = cpuid & 0xffff;
We can nicely oneline it, but that's about all the benefits I can see.
It is prone to typos, hard to search for and limiting our operations to
a simple assignment to a properly named variable.
(I prefer descriptive, horribly long, names to raw constant everywhere,
MASK_MAX_MONITOR_LINE_SIZE_FAM_X.)
Second problem: Most elements don't begin at offset 0, so the usual
retrieval would add a shift, (repurposing max_monitor_line_size)
max_monitor_line_size = (cpuid & MASK_FAM_X) >> OFFSET_FAM_X;
and it's not better when we write it back after doing stuff.
cpuid = (cpuid & ~MASK_FAM_X) | (max_monitor_line_size << OFFSET_FAM_X
& MASK_FAM_X);
All would be fine if we abstracted this with more macros ... wait,
bitfield already does that!
max_monitor_line_size = cpuid.split.max_monitor_line_size_fam_x;
cpuid.split.max_monitor_line_size_fam_x = max_monitor_line_size;
---
OT: I'd remove '.split', but we probably wouldn't agree about '.full'.
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