On 4 April 2018 at 15:28, James Morse <james.morse@xxxxxxx> wrote: > Hi Kostiantyn, > > On 04/04/18 13:45, Kostiantyn Iarmak wrote: >> From: Pratyush Anand <panand@xxxxxxxxxx> >>> Date: Fri, Jun 2, 2017 at 5:42 PM >>> Subject: Re: [PATCH v3 0/2] kexec-tools: arm64: Enable D-cache in purgatory >>> To: James Morse <james.morse@xxxxxxx> >>> Cc: mark.rutland@xxxxxxx, bhe@xxxxxxxxxx, kexec@xxxxxxxxxxxxxxxxxxx, >>> horms@xxxxxxxxxxxx, dyoung@xxxxxxxxxx, >>> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx >>> >>> On Friday 02 June 2017 01:53 PM, James Morse wrote: >>>> On 23/05/17 06:02, Pratyush Anand wrote: >>>>> It takes more that 2 minutes to verify SHA in purgatory when vmlinuz image >>>>> is around 13MB and initramfs is around 30MB. It takes more than 20 second >>>>> even when we have -O2 optimization enabled. However, if dcache is enabled >>>>> during purgatory execution then, it takes just a second in SHA >>>>> verification. >>>>> >>>>> Therefore, these patches adds support for dcache enabling facility during >>>>> purgatory execution. > >>>> I'm still not convinced we need this. Moving the SHA verification to happen >>>> before the dcache+mmu are disabled would also solve the delay problem, >>> >>> Humm..I am not sure, if we can do that. > >>> When we leave kernel (and enter into purgatory), icache+dcache+mmu are >>> already disabled. I think, that would be possible when we will be in a >>> position to use in-kernel purgatory. >>> >>>> and we >>>> can print an error message or fail the syscall. >>>> >>>> For kexec we don't expect memory corruption, what are we testing for? >>>> I can see the use for kdump, but the kdump-kernel is unmapped so the kernel >>>> can't accidentally write over it. >>>> >>>> (we discussed all this last time, but it fizzled-out. If you and the >>>> kexec-tools maintainer think its necessary, fine by me!) > >>> Yes, there had already been discussion and MAINTAINERs have >>> discouraged none-purgatory implementation. >>> >>>> I have some comments on making this code easier to maintain.. >>>> >>> Thanks. >>> >>> I have implemented your review comments and have archived the code in >>> >>> https://github.com/pratyushanand/kexec-tools.git : purgatory-enable-dcache >>> >>> I will be posting the next version only when someone complains about >>> ARM64 kdump behavior that it is not as fast as x86. > >> On our ARM64-based platform we have very long main kernel-secondary kernel >> switch time. >> >> This patch set fixes the issue (we are using 4.4 kernel and 2.0.13 kexec-tools >> version), we can get ~25x speedup, with this patch secondary kernel boots in ~3 >> seconds while on 2.0.13-2.0.16 kexec-tools without this patch switch takes about >> 75 seconds. > > This is slow because its generating a checksum of the kernel without the benefit > of the caches. This series generated page tables so that it could enable the MMU > and caches. But, the purgatory code also needs to be a simple as possible > because its practically impossible to debug. > > The purgatory code does this checksum-ing because its worried the panic() was > because the kernel cause some memory corruption, and that memory corruption may > have affected the kdump kernel too. > If this is the only reason, there is no need to use a strong cryptographic hash, and we should be able to recover some performance by switching to CRC32 instead, preferably using the special arm64 instructions (if implemented). But I agree that skipping the checksum calculation altogether is probably the best approach here. > This can't happen on arm64 as we unmap kdump's crash region, so not even the > kernel can accidentally write to it. 98d2e1539b84 ("arm64: kdump: protect crash > dump kernel memory") has all the details. > > (we also needed to do this to avoid the risk of mismatched memory attributes if > kdump boots and some CPUs are still stuck in the old kernel) > > >> When do you plan merge this patch? > > We ended up with the check-summing code because its the default behaviour of > kexec-tools on other architectures. > > One alternative is to rip it out for arm64. Untested: > --------------------%<-------------------- > diff --git a/purgatory/arch/arm64/Makefile b/purgatory/arch/arm64/Makefile > index 636abea..f10c148 100644 > --- a/purgatory/arch/arm64/Makefile > +++ b/purgatory/arch/arm64/Makefile > @@ -7,7 +7,8 @@ arm64_PURGATORY_EXTRA_CFLAGS = \ > -Werror-implicit-function-declaration \ > -Wdeclaration-after-statement \ > -Werror=implicit-int \ > - -Werror=strict-prototypes > + -Werror=strict-prototypes \ > + -DNO_SHA_IN_PURGATORY > > arm64_PURGATORY_SRCS += \ > purgatory/arch/arm64/entry.S \ > diff --git a/purgatory/purgatory.c b/purgatory/purgatory.c > index 3bbcc09..44e792a 100644 > --- a/purgatory/purgatory.c > +++ b/purgatory/purgatory.c > @@ -9,6 +9,8 @@ > struct sha256_region sha256_regions[SHA256_REGIONS] = {}; > sha256_digest_t sha256_digest = { }; > > +#ifndef NO_SHA_IN_PURGATORY > + > int verify_sha256_digest(void) > { > struct sha256_region *ptr, *end; > @@ -39,14 +41,18 @@ int verify_sha256_digest(void) > return 0; > } > > +#endif /* NO_SHA_IN_PURGATORY */ > + > void purgatory(void) > { > printf("I'm in purgatory\n"); > setup_arch(); > +#ifndef NO_SHA_IN_PURGATORY > if (verify_sha256_digest()) { > for(;;) { > /* loop forever */ > } > } > +#endif /* NO_SHA_IN_PURGATORY */ > post_verification_setup_arch(); > } > --------------------%<-------------------- > > > Thanks, > > James > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
