Re: [PATCH v8 02/13] kexec_file: make an use of purgatory optional

[Dave Young]

On 02/26/18 at 07:24pm, AKASHI Takahiro wrote:
> On Fri, Feb 23, 2018 at 04:49:34PM +0800, Dave Young wrote:
> > Hi AKASHI,
> > 
> > On 02/22/18 at 08:17pm, AKASHI Takahiro wrote:
> > > On arm64, no trampline code between old kernel and new kernel will be
> > > required in kexec_file implementation. This patch introduces a new
> > > configuration, ARCH_HAS_KEXEC_PURGATORY, and allows related code to be
> > > compiled in only if necessary.
> > 
> > Here also need the explanation about why no purgatory is needed, it would be
> > required for kexec if no strong reason.
> 
> OK, I will add the reason:
> On arm64, crash dump kernel's usable memory is protected by
> *unmapping* it from kernel virtual space unlike other architectures
> where the region is just made read-only.
> So our key developers think that it is highly unlikely that the region
> is accidentally corrupted and this rationalizes that digest check code
> be also dropped from purgatory.
> This greatly simplifies our purgatory without any need for a bit ugly
> relocation stuff, i.e. arch_kexec_apply_relocations_add().
> 
> Please see:
>    http://lists.infradead.org/pipermail/linux-arm-kernel/2017-December/545428.html
> to find out how simple our purgatory was. All that it does is
> to shuffle arguments and jump into a new kernel.
> 
> Without this patch, we would have to have purgatory with a space for
> a hash value (purgatory_sha256_digest) which is never checked against.
> 
> Do you think it makes sense?

Hmm, it looks reasonable, I remember there could be some performance
issue for a purgatory because of cache disabled for arm64. I do not
object this.

[snip]

Thanks
Dave