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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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