RE: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter_mc()

[David Laight <David.Laight@ACULAB.COM>]

From: David Howells
> Sent: Friday, August 18, 2023 12:43 PM
> 
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > Well, that part is trivially fixable, and we should do that anyway for
> > other reasons.
> > ..
> >  enum iter_type {
> >  	/* iter types */
> > -	ITER_IOVEC,
> > -	ITER_KVEC,
> > -	ITER_BVEC,
> > -	ITER_XARRAY,
> > -	ITER_DISCARD,
> > -	ITER_UBUF,
> > +	ITER_IOVEC = 1,
> > +	ITER_UBUF = 2,
> > +	ITER_KVEC = 4,
> > +	ITER_BVEC = 8,
> > +	ITER_XARRAY = 16,
> > +	ITER_DISCARD = 32,

That could be zero - no bits and default.

> >  };
> 
> It used to be this way, but Al switched it:
> 
> 	8cd54c1c848031a87820e58d772166ffdf8c08c0
> 	iov_iter: separate direction from flavour

Except it also had the direction flag inside the enum.
That caused its own piles of grief.

IIRC Linus had type:6 - that doesn't leave any headroom
for additional types (even though they shouldn't proliferate).
It may be best to avoid bits 15+ (in a bitfield) due to
issues with large constants and sign extension.
On x86 (I think) 'and immediate' and 'bit test' are the same
size for bit 0 to 7, BIT wins for higher bits.

gcc generates strange code for some initialisers (see yesterday's
thread) and you definitely mustn't leave unused bits in a bitfield.
Might be better is the fields are assigned later!

(I also saw clang carefully preserving %eax on stabck!)

	David

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