From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <ian.campbell@citrix.com>
Subject: Re: [PATCH v1 2/4] xen-version: Add third parameter
 (len) to the do_version hypercall.
Date: Fri, 9 Oct 2015 13:46:45 +0100
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To: Andrew Cooper <andrew.cooper3@citrix.com>, Jan Beulich <JBeulich@suse.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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On Fri, 2015-10-09 at 13:29 +0100, Andrew Cooper wrote:
> On 09/10/15 09:25, Jan Beulich wrote:
> > > > > On 09.10.15 at 04:56, <konrad.wilk@oracle.com> wrote:
> > > All existing commands ignore the parameter so this does
> > > not break the ABI.
> > Does it not? What about the debug mode clobbering of hypercall
> > argument registers?
> 
> That is an implementation detail of the hypervisor.  It is irrelevant to
> guests whether Xen chooses to clobber the spare registers or not.

Or in other words the effect here is to clobber one _less_ register, and
the guest cannot have been relying on a register getting so clobbered (if
nothing else it doesn't happen in debug=n builds).

The flip side is that we are now no longer clobbering that register even
for existing sub-ops which do not use it (since the clobbering doesn't go
down to the subop level). So there is a risk that a guest may come to
depend on that register not being clobbered and then fail older debug=y
hypervisors.

This second scenario doesn't seem especially likely to me.

Do we not already have one or two hypercalls where subops consume different
numbers of parameters anyway? HYPERVISOR_sched_op I think has this property
and we've not been too concerned.

Ian.