Re: [Xen-devel] [PATCH] xen: only clobber multicall elements without error

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Jan Beulich writes ("Re: [PATCH] xen: only clobber multicall elements without 
error"):
> On 23.11.18 at 14:25, <jgross@xxxxxxxx> 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.

Ian.

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