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Re: [PATCH v2 09/13] MIPS: Add functions for hypervisor call

To: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Subject: Re: [PATCH v2 09/13] MIPS: Add functions for hypervisor call
From: Ralf Baechle <ralf@linux-mips.org>
Date: Tue, 3 Jun 2014 10:30:31 +0200
Cc: linux-mips@linux-mips.org, David Daney <ddaney.cavm@gmail.com>, James Hogan <james.hogan@imgtec.com>, kvm@vger.kernel.org, David Daney <david.daney@cavium.com>
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On Wed, May 28, 2014 at 11:52:12PM +0200, Andreas Herrmann wrote:

> +/*
> + * Hypercalls for KVM.
> + *
> + * Hypercall number is passed in v0.
> + * Return value will be placed in v0.
> + * Up to 3 arguments are passed in a0, a1, and a2.
> + */
> +static inline unsigned long kvm_hypercall0(unsigned long num)
> +{
> +     register unsigned long n asm("v0");
> +     register unsigned long r asm("v0");

Btw, is it safe to put two variables in the same register?

The syscall wrappers that used to be in <asm/unistd.h> were occasionally
hitting problems which eventually forced me to stop forcing variables
into particular registers instead using a MOVE instruction to shove
each variable into the right place.

Of course they were being used from non-PIC and PIC code, kernel and userland
so GCC had a much better chance to do evil than in the hypercall wrapper
case - but it made me paranoid ...

  Ralf

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