Re: [Xen-devel] [PATCH v1 3/8] xen/arm: Support sign-extension for every read access

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Mon, 2015-09-28 at 19:22 +0100, Julien Grall wrote:
> On 25/09/15 17:44, Ian Campbell wrote:
> > On Fri, 2015-09-25 at 15:51 +0100, Julien Grall wrote:
> > I think you meant s/bit sign/sign bit/ but more correct would be "Sign
> > extend if required".
> > 
> > > +     * Note that we expect the read handler to have zeroed the bit
> > > +     * unused in the register.
> > 
> > "... to have zeroed the unused bits in the register".
> > 
> > But I think "unused" is a bit misleading, you mean the ones outside the
> > requested access size, those bits are still "used" IYSWIM. I can't
> > think of
> > a terse term for "outside the requested access size I'm afraid.
> 
> I will switch to "Note that we expect the read handler to have zeroed
> the bits outside the requested access size."
> 
> > 
> > Did you confirm that all existing handlers meet this requirement?
> 
> Yes, we always do *r in the existing handlers.
> 
> > Perhaps an ASSERT would be handy?
> 
> What about:
> 
> ASSERT((*r & ((~0UL) >> (BITS_PER_LONG - size))) == 0)

Is that not backwards, e.g for size = 8, then 

~0UL >> (32-8) == 0xffffffff >> 24 == 0xff, so you end up checking that the
lowest byte is zero, but that's the one you expected to change.

Or is it, couldn't we be updating a byte in the middle of the word?

Probably figuring out the correct assertion is more hassle than it is
worth..


> 
> Regards,
> 

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