Re: Hypercalls: ignored output registers?

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 27/12/2015 03:42, Carl Patenaude Poulin wrote:
> Hi all,
>
> I'm looking at how hypercalls are implemented in Mini-OS and Andrew
> Cooper's xen-test-framework. In both cases, several registers are
> defined as outputs, they're bound by input registers, and their
> outputs are ignored. Is there any reason for doing this instead of
> just declaring directly the input registers?
>
> Is it because the registers are callee-saved? If that's the case,
> why not just put them in the clobber list?

This is because of some of the restrictions when constructing asm 
statements.

The clobber list must be mutually exclusive with any of the named 
inputs/ouputs.  Anything listed in the inputs alone is expected to 
retain its value across the asm statement.

As the values in the registers are modified by the hypercall, we need a 
separate way to indicate that they are clobbered.

This is done by listing them as an output as well (which indicates that 
their value is now different to the value they had as an input), being 
directed at a variable which is never referenced again (i.e. we don't 
care what the eventual value actually is).

~Andrew

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