Re: [Xen-devel] [PATCH] xen/arm64: disable alignment check

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-04-28 at 11:24 +0100, David Vrabel wrote:
> On 28/04/14 10:48, Ian Campbell wrote:
> > On Sun, 2014-04-27 at 10:10 +0100, Vladimir Murzin wrote:
> >> Alignment check is enabled by default at Xen boot.
> > 
> > This has already been disabled in the development branch via:
> > commit 58bbe7d71239db508c30099bf7b6db7c458f3336
> > Author: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Date:   Wed Mar 26 13:38:45 2014 +0000
> > 
> >     xen: arm64: disable alignment traps
> >     
> >     The mem* primitives which I am about to import from Linux in a 
> > subsequent
> >     patch rely on the hardware handling misalignment.
> >     
> >     The benefits of an optimised memcpy etc outweigh the downsides.
> >     
> >     Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> >     Acked-by: Julien Grall <julien.grall@xxxxxxxxxx>
> >     Acked-by: Tim Deegan <tim@xxxxxxx>
> > 
> > I will consider this for backport, but first I'd like to consider
> > whether we shouldn't fix the hypervisor side evtchn FIFO code along the
> > same lines as the kernel side. David, any thoughts?
> 
> I believe Jan suggested making Xen's bitops handle 32-bit alignment or
> adding a new set of 32-bit bitops.

This is already the case for the arm64 bitops, we deliberately diverged
from Linux here because there are a bunch of other "misaligned" 4-byte
bitmasks (one in the malloc implementation springs to mind).

Ian.



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