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

From: Alex Williamson <alex.williamson@hp.com>
To: "Yang, Sheng" <sheng.yang@intel.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: [PATCH] kvm: Use a bitmap for tracking used GSIs
Date: Tue, 12 May 2009 12:45:36 -0600	[thread overview]
Message-ID: <1242153936.23680.131.camel@bling> (raw)
In-Reply-To: <200905112000.10771.sheng.yang@intel.com>

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,

Alex