[PATCH 1/2] arm64: Add enable/disable d-cache support for purgatory

[Mark Rutland]

On Wed, Dec 14, 2016 at 11:16:17AM +0000, James Morse wrote:
> Hi Pratyush,
> 
> On 14/12/16 10:12, Pratyush Anand wrote:
> > On Wednesday 14 December 2016 03:08 PM, Pratyush Anand wrote:
> >>> I would go as far as to generate the page tables at 'kexec -l' time,
> >>> and only if
> >>
> >> Ok..So you mean that I create a new section which will have page table
> >> entries mapping physicalmemory represented by remaining section, and
> >> then purgatory can just enable mmu with page table from that section,
> >> right? Seems doable. can do that.
> > 
> > I see a problem here. If we create  page table as a new segment then, how can we
> > verify in purgatory that sha for page table is correct? We need page table
> > before sha verification start,and we can not rely the page table created by
> > first kernel until it's sha is verified. So a chicken-egg problem.
> 
> There is more than one of those! What happens if your sha256 calculation code is
> corrupted? You have to run it before you know. The same goes for all the
> purgatory code.
> 
> This is why I think its better to do this in the kernel before we exit to
> purgatory, but obviously that doesn't work for kdump.

I see in an earlier message that the need for sha256 was being discussed
in another thread. Do either of you happen to have a pointer to that.

To me, it seems like it doesn't come with much benefit for the kdump
case given that's best-effort anyway, and as above the verification code
could have been be corrupted. In the non-kdump case it's not strictly
necessary and seems like a debugging aid rather than a necessary piece
of functionality -- if that's the case, a 20 second delay isn't the end
of the world...

Thanks,
Mark.