Re: [Xen-devel] [PATCH 04/19] xen: arm: provide and use a handle_raz_wi helper

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 02/04/2015 18:19, Ian Campbell wrote:
> On Thu, 2015-04-02 at 17:01 +0100, Ian Campbell wrote:
> > On Thu, 2015-04-02 at 16:50 +0100, Ian Campbell wrote:
> >
> > > Writing to the bottom half (e.g. w0) of a register implicitly clears the
> > > top half, IIRC, so I think a kernel is unlikely to want to do this, even
> > > if it could (which I'm not quite convinced of).
> > That said, I'll see if I can make something work with the handle_*
> > taking the reg number instead of a pointer and calling select_user_reg
> > in each.
> Actually don't even need that, I think the following does what is
> needed. I'm not 100% convinced it is needed though, but it's simple
> enough, and I can't find anything in the ARM ARM right now which rules
> out what you are suggesting, even if it is unlikely.
The paragraph "Pseudocode description of registers in AArch64 state" in 
section B1.2.1 (ARMv8 DDI0487 A.d) confirms your previous mail. I.e 
"writing to the bottom half (e.g. w0) of a register implicitly clears 
the top half".
I think it may be worth to mention the paragraph somewhere in the patch.

Regards,

--
Julien Grall

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