Re: [PATCH -v3] KVM: x86: lapic: Clean up find_highest_vector() and count_vectors()

[Takuya Yoshikawa <takuya.yoshikawa@gmail.com>]

On Thu, 30 Aug 2012 16:21:31 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:
 
> > +static u32 apic_read_reg(int reg_off, void *bitmap)
> > +{
> > +	return *((u32 *)(bitmap + reg_off));
> > +}
> > +
> 
> Contrast with apic_set_reg which gets apic,
> add fact that all callers invoke REG_POS and you will
> see this is a bad API.
> 
> I played with some APIs but in the end it's
> probably better to just open-code this.

I don't mind open-coding this.

> As a bonus, open-coding will avoid the need
> for cast above, which is good: casts make code more
> fragile.

But I still don't understand why we can eliminate casting:

  u32 reg_val;

  reg_val = *((u32 *)(bitmap + REG_POS(vec)));
  if (reg_val)
      return __fls(reg_val) + vec;

(I'm not sure compilers are allowed to push out the value and
do multiple references for this code as explained in
https://lwn.net/Articles/508991/ )


If you mean

  u32 *reg;

  reg = bitmap + REG_POS(vec);
  if (*reg)
      return __fls(*reg) + vec;

I'm still not confident if this is a good style.
I rarely see code doing

  if (*p)
      __fls(*p);

This looks like explicite multiple references: I'm not saying
this will actually be compiled to do multiple references.

Thanks,
	Takuya