Re: [Xen-devel] [PATCH 02/17] xen/arm64: head: Don't clobber x30/lr in the macro PRINT

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 25 Jun 2019, Stefano Stabellini wrote:
> On Mon, 10 Jun 2019, Julien Grall wrote:
> >>  The current implementation of the macro PRINT will clobber x30/lr. This
> > means the user should save lr if it cares about it.
> 
> By x30/lr, do you mean x0-x3 and lr? I would prefer a clearer
> expression.

No of course not! You meant x30 which is a synonym of lr! It is just
that in this case it is also supposed to clobber x0-x3 -- I got
confused! The commit message is also fine as is then. More below.


> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> 
> 
> > Follow-up patches will introduce more use of PRINT in place where lr
> > should be preserved. Rather than requiring all the users to preserve lr,
> > the macro PRINT is modified to save and restore it.
> > 
> > While the comment state x3 will be clobbered, this is not the case. So
> > PRINT will use x3 to preserve lr.
> > 
> > Lastly, take the opportunity to move the comment on top of PRINT and use
> > PRINT in init_uart. Both changes will be helpful in a follow-up patch.
> > 
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > ---
> >  xen/arch/arm/arm64/head.S | 14 +++++++++-----
> >  1 file changed, 9 insertions(+), 5 deletions(-)
> > 
> > diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> > index c8bbdf05a6..a5147c8d80 100644
> > --- a/xen/arch/arm/arm64/head.S
> > +++ b/xen/arch/arm/arm64/head.S
> > @@ -78,12 +78,17 @@
> >   *  x30 - lr
> >   */
> >  
> > -/* Macro to print a string to the UART, if there is one.
> > - * Clobbers x0-x3. */
> >  #ifdef CONFIG_EARLY_PRINTK
> > +/*
> > + * Macro to print a string to the UART, if there is one.
> > + *
> > + * Clobbers x0 - x3
> > + */
> >  #define PRINT(_s)           \
> > +        mov   x3, lr  ;     \
> >          adr   x0, 98f ;     \
> >          bl    puts    ;     \
> > +        mov   lr, x3  ;     \
> >          RODATA_STR(98, _s)

Strangely enough I get a build error with gcc 7.3.1, but if I use x30
instead of lr, it builds fine. Have you seen this before?
The error is:

arm64/head.S: Assembler messages:
arm64/head.S:272: Error: operand 1 must be an integer register -- `mov lr,x3'
[...]
arm64/head.S:272: Error: undefined symbol lr used as an immediate value
[...]

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