Re: Re: [PATCH] kvm: Use a bitmap for tracking used GSIs

[Alex Williamson <alex.williamson@hp.com>]

On Tue, 2009-05-12 at 12:45 -0600, Alex Williamson wrote:
> On Mon, 2009-05-11 at 12:00 +0000, Yang, Sheng wrote:
> > On Friday 08 May 2009 06:22:20 Alex Williamson wrote:
> > > +		/* Round the number of GSIs supported to a 4 byte
> > > +		 * value so we can search it using ints and ffs */
> > > +		i = kvm_get_gsi_count(kvm) & ~0x1f;
> > > +		kvm->used_gsi_bitmap = malloc(i >> 3);
> > 
> > 3 or 5?
> 
> 3, ie. /8 (bits to bytes)
> 
> > I am a little confused by these magic numbers, including 0x1f...
> 
> The 5 shift gives us the index into the array of ints, the 0x1f gives us
> the bit index into a specific int.  This is very similar to the code in
> hw/acpi.c.
> 
> > I think there are something can indicate the length of unsigned long in 
> > QEmu(sorry, can't find it now...), so how about using ffsl() and get other 
> > constants based on it?
> 
> We'd probably want to use ffsll() so we can ignore 32b vs 64b longs.
> There's HOST_LONG_BITS, but that doesn't actually help defining a shift
> value.  Thanks,

Hmm, neither ffsl() or ffsll() are standard.  I'm inclined to stick with
32bits.  We'll likely only be using the first index on x86_64 anyway
(2nd on ia64).  Thanks,

Alex