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

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Alex Williamson <alex.williamson@hp.com>
Cc: "Yang, Sheng" <sheng.yang@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: Re: [PATCH] kvm: Use a bitmap for tracking used GSIs
Date: Tue, 12 May 2009 22:10:53 +0300	[thread overview]
Message-ID: <20090512191053.GA28935@redhat.com> (raw)
In-Reply-To: <1242155214.23680.151.camel@bling>

On Tue, May 12, 2009 at 01:06:54PM -0600, Alex Williamson wrote:
> 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

With MSI, we start with GSI 24, so we'll be using more than just the
first index.

-- 
MST