Re: [PATCH 2/3] arm64: KVM: Allow unaligned accesses at EL2

[Christoffer Dall <cdall@linaro.org>]

On Wed, Jun 07, 2017 at 10:16:29AM +0100, Marc Zyngier wrote:
> On 06/06/17 21:09, Christoffer Dall wrote:
> > On Tue, Jun 06, 2017 at 07:08:34PM +0100, Marc Zyngier wrote:
> >> We currently have the SCTLR_EL2.A bit set, trapping unaligned accesses
> >> at EL2, but we're not really prepared to deal with it. So far, this
> >> has been unnoticed, until GCC 7 started emitting those (in particular
> >> 64bit writes on a 32bit boundary).
> >>
> >> Since the rest of the kernel is pretty happy about that, let's follow
> >> its example and set SCTLR_EL2.A to zero. Modern CPUs don't really
> >> care.
> > 
> > Why do we set the A flag via SCTLR_ELx_FLAGS in the first place, only to
> > drop that flag later on for both EL1 and EL2 ?
> 
> That flag is always cleared at EL1, never set. Actually, only EL2 uses
> that macro to *set* flags. An alternative would be to do away with the
> macro and use the individual flags, like the 32bit side does.
> 
> What do you think?
> 
I don't understand why the A bit is part of SCTLR_ELx_FLAGS then?  Is it
used as a mask, is that why?

In terms of these patches, I think we should apply these, because they
solve the problem and do the same thing.

Thanks,
-Christoffer