Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

Peter Zijlstra
2020-09-30T17:14:04Z

On Wed, Sep 30, 2020 at 11:10:36AM -0500, Segher Boessenkool wrote:

> Since this variable is a local register asm, on entry to the asm the
> compiler guarantees that the value lives in the assigned register (the
> "r8" hardware register in this case). This all works completely fine.
> This is the only guaranteed behaviour for local register asm (well,
> together with analogous behaviour for outputs).

Right, that's what they're trying to achieve. The hypervisor calling
convention needs that variable in %r8 (which is somewhat unfortunate).

AFAIK this is the first such use in the kernel, but at least the gcc-4.9
(our oldest supported version) claims to support this.

So now we need to know if clang will actually do this too..