Re: [Xen-devel] [PATCH 2/2] xen/arm64: use shift operator

[Julien Grall <julien.grall@xxxxxxx>]

On 21/04/16 02:06, Peng Fan wrote:
> Hi Julien,
Hello Peng,

> On Wed, Apr 20, 2016 at 03:44:09PM +0100, Julien Grall wrote:
> > Hello Peng,
> >
> > On 20/04/16 14:54, Peng Fan wrote:
> > > Use shift operator, but not muliplication.
> > > No function change.
> > Why? The compiler will calculate the address at compilation time.
> Yeah. The compiler will do this. I just think in asm, we rarely use
> multiplication. In this file, there is "cmp   x1, #(LPAE_ENTRIES<<3)",
> here use shift operator but not multiplication.
It took me a bit of time to understand that 8 is the number of byte of 
an LPAE entries. IMHO the "<< 3" would be more confusing.
So I would introduce a macro to get the slot based of an index. 
Something like:
LPAE_SLOT(slot) ((slot) * 8)

And add a comment to explain why 8. You could even introduce 
LPAE_ENTRY_SIZE.
Regards,

> Thanks,
> Peng.
>
> > Regards,
> >
> > > Signed-off-by: Peng Fan <van.freenix@xxxxxxxxx>
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Cc: Julien Grall <julien.grall@xxxxxxx>
> > > ---
> > >   xen/arch/arm/arm64/head.S | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> > > index 05e3db0..ad6e593 100644
> > > --- a/xen/arch/arm/arm64/head.S
> > > +++ b/xen/arch/arm/arm64/head.S
> > > @@ -524,7 +524,7 @@ paging:
> > >           lsl   x2, x2, #THIRD_SHIFT   /* 4K aligned paddr of UART */
> > >           mov   x3, #PT_DEV_L3
> > >           orr   x2, x2, x3             /* x2 := 4K dev map including UART */
> > > -        str   x2, [x1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's 
> > > slot */
> > > +        str   x2, [x1, #(FIXMAP_CONSOLE << 3)] /* Map it in the first fixmap's 
> > > slot */
> > >   1:
> > >
> > >           /* Map fixmap into boot_second */
> > --
> > Julien Grall
--
Julien Grall

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