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