Re: [Xen-devel] [PATCH v1 2/4] xen-version: Add third parameter (len) to the do_version hypercall.

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-10-09 at 08:38 -0600, Jan Beulich wrote:
> > > > On 09.10.15 at 14:46, <ian.campbell@xxxxxxxxxx> wrote:
> > 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@xxxxxxxxxx> 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).
> 
> No, the one less register clobbered is in the first clobbering phase,
> where _unused_ inputs get clobbered (for hypervisor internal
> consumption). The second clobbering phase destroys all _used_
> input registers' contents (the guest visible values), and _this_ is
> what results in ABI breakage (because callers assuming the
> hypercall to take two arguments assume that the 3rd argument
> register will retain its contents.

Ah, yes, that's correct. My mistake.

Ian.

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