On 26/11/2018 15:54, Ian Jackson wrote:
> Jan Beulich writes ("Re: [PATCH] xen: only clobber multicall elements without
> error"):
>> On 23.11.18 at 14:25, <[email protected]> wrote:
>>> In debug builds the hypervisor will deliberately clobber processed
>>> elements of the multicall structure. In order to ease diagnostic data
>>> printout in the affected guest only clobber elements which didn't
>>> return an error.
>>
>> Besides what Andrew has said such a relaxation reduces
>> the guarding against bad guest side code. If a guest really
>> wishes to produce diagnostics, I think it should go to the
>> lengths of copying arguments (if they can't be re-calculated
>> anyway). Suppressing the clobbering in more cases merely
>> invites guests to read the arguments after the call, which
>> they simply should not do. Not clobbering the values in
>> release builds is a performance choice, and we ought to be
>> allowed to change our opinion regarding this implementation
>> detail at any point in time.
>
> Maybe they could be clobbered losslessly ? Eg, by xoring with 0xaa or
> something.
This would be rather hacky: I'd need to find out if clobbering was
performed or not and the way of clobbering would be kind of an interface
which I guess we'd like to avoid.
Juergen
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