Re: [Xen-devel] [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 29 Feb 2012, Dave Martin wrote:
> On Wed, Feb 29, 2012 at 09:56:02AM +0000, Ian Campbell wrote:
> > On Wed, 2012-02-29 at 09:34 +0000, Dave Martin wrote:
> > > On Tue, Feb 28, 2012 at 12:28:29PM +0000, Stefano Stabellini wrote:
> > 
> > > > I don't have a very strong opinion on which register we should use, but
> > > > I would like to avoid r7 if it is already actively used by gcc.
> > > 
> > > But there is no framepointer for Thumb-2 code (?)
> > 
> > Peter Maydell suggested there was:
> > > r7 is (used by gcc as) the Thumb frame pointer; I don't know if this
> > > makes it worth avoiding in this context.
> > 
> > Sounds like it might be a gcc-ism, possibly a non-default option?
> > 
> > Anyway, I think r12 will be fine for our purposes so the point is rather
> > moot.
> 
> Just had a chat with some tools guys -- apparently, when passing register
> arguments to gcc inline asms there really isn't a guarantee that those
> variables will be in the expected registers on entry to the inline asm.
> 
> If gcc reorders other function calls or other code around the inline asm
> (which it can do, except under certain controlled situations), then
> intervening code can clobber any registers in general.
> 
> Or, to summarise another way, there is no way to control which register
> is used to pass something to an inline asm in general (often we get away
> with this, and there are a lot of inline asms in the kernel that assume
> it works, but the more you inline the more likely you are to get nasty
> surprises).  There is no workaroud, except on some architectures where
> special asm constraints allow specific individual registers to be
> specified for operands (i386 for example).
> 
> If you need a specific register, this means that you must set up that
> register explicitly inside the asm if you want a guarantee that the
> code will work:
> 
>       asm volatile (
>               "movw   r12, %[hvc_num]\n\t"
>               ...
>               "hvc    #0"
>               :: [hvc_num] "i" (NUMBER) : "r12"
>       );
> 

OK, we can arrange the hypercall code to be like that.
Also with your patch series it would be "_hvc" because of the .macro,
right?



> This is the kind of problem which goes away when out-of-lining the
> hvc wrapper behind a C function interface, since the ABI then provides
> guarantees about how values are mershaled into and out of that code.

Do you mean implementing the entire HYPERVISOR_example_op in assembly
and calling it from C?
Because I guess that gcc would still be free to mess with the registers
between the C function entry point and any inline assembly code.

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