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

From: AKASHI Takahiro <takahiro.akashi@linaro.org>
To: Dave Young <dyoung@redhat.com>
Cc: catalin.marinas@arm.com, will.deacon@arm.com,
	bauerman@linux.vnet.ibm.com, dhowells@redhat.com,
	vgoyal@redhat.com, herbert@gondor.apana.org.au,
	davem@davemloft.net, akpm@linux-foundation.org,
	mpe@ellerman.id.au, bhe@redhat.com, arnd@arndb.de,
	ard.biesheuvel@linaro.org, julien.thierry@arm.com,
	kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 02/13] kexec_file: make an use of purgatory optional
Date: Thu, 1 Mar 2018 11:59:22 +0900	[thread overview]
Message-ID: <20180301025921.GL6019@linaro.org> (raw)
In-Reply-To: <20180228123359.GB2228@dhcp-128-65.nay.redhat.com>

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