Re: [Xen-devel] [PATCH] xen/kexec: Clear unused registers before jumping into an image

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Mon, Nov 18, 2013 at 11:23:27AM +0000, David Vrabel wrote:
> On 18/11/13 09:29, Jan Beulich wrote:
> >>>> On 15.11.13 at 21:07, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> >> On 15/11/13 15:56, Daniel Kiper wrote:
> >>> Clear unused registers before jumping into an image. This way
> >>> loaded image could not assume that any register has an specific
> >>> info about earlier running Xen hypervisor. However, it also
> >>> does not mean that the image may expect that a given register
> >>> is zeroed. The image MUST assume that every register has a random
> >>> value or in other words it is uninitialized or has undefined state.
> >>
> >> I think this, where the specification (registers undefined) differs from
> >> the implementation (registers zeroed), is the worst option.
> >>
> >> I also think it is more likely for an image to inadvertently rely on a
> >> zero value that whatever junk Xen has left behind.
> >
> > Preventing users to rely on anything would likely make it
> > desirable to put some random value into all unused registers.
>
> I don't think we need to go that far.
>
> I would just like to avoid someone looking that the implementation (and
> not the documentation) and concluding that zero-ing of the registers is
> part of the specified behaviour, or looking at the implementation and
> documentation and wondering why they don't agree.

David, my comment clearly states why we are doing that and what should
be expected. What is wrong with it? I could improve it but say how?

Daniel

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