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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-04-03 at 14:39 +0200, Julien Grall wrote:
> 
> 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.

Yes, I shall, in the commit log most likely.

Ian.


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