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

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Date:   Thu, 1 Mar 2018 11:59:22 +0900
From:   AKASHI Takahiro <takahiro.akashi@...aro.org>
To:     Dave Young <dyoung@...hat.com>
Cc:     catalin.marinas@....com, will.deacon@....com,
        bauerman@...ux.vnet.ibm.com, dhowells@...hat.com,
        vgoyal@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, akpm@...ux-foundation.org, mpe@...erman.id.au,
        bhe@...hat.com, arnd@...db.de, ard.biesheuvel@...aro.org,
        julien.thierry@....com, kexec@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 02/13] kexec_file: make an use of purgatory optional

On Wed, Feb 28, 2018 at 08:33:59PM +0800, Dave Young wrote:
> 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.

Yeah, Pratyush(redhat) had expressed his concerns on slow boot-up of
the 2nd kernel which is due to hash value calculation.

-Takahiro AKASHI

> 
> [snip]
> 
> Thanks
> Dave