Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

[Eric DeVolder]

On 10/4/22 04:10, Sourabh Jain wrote:
> On 30/09/22 21:06, Eric DeVolder wrote:
> > On 9/28/22 11:07, Borislav Petkov wrote:
> > > On Tue, Sep 13, 2022 at 02:12:31PM -0500, Eric DeVolder wrote:
> > > > This topic was discussed previously https://lkml.org/lkml/2022/3/3/372.
> > >
> > > Please do not use lkml.org to refer to lkml messages. We have a
> > > perfectly fine archival system at lore.kernel.org. You simply do
> > >
> > > https://lore.kernel.org/r/<Message-ID>
> > >
> > > when you want to point to a previous mail.
> >
> > ok, thanks for pointing that out to me.
> > > > David points out that terminology is tricky here due to differing behaviors.
> > > > And perhaps that is your point in asking for guidance text. It can be
> > > > complicated
> > >
> > > Which means you need an explanation how to use this even more.
> > >
> > > And why is CONFIG_CRASH_MAX_MEMORY_RANGES even a Kconfig item and not
> > > something you discover from the hardware?
> >
> > No, is the short answer.
> >
> > > Your help text talks about System RAM entries in /proc/iomem which means
> > > that those entries are present somewhere in the kernel and you can read
> > > them out and do the proper calculations dynamically instead of doing the
> > > static CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES thing.
> >
> > The intent is to compute the max size buffer needed to contain a maximum populated elfcorehdr, 
> > which is primarily based on the number of CPUs and memory regions. Thus far I (and others 
> > involved) have not found a kernel method to determine the maximum number of memory regions 
> > possible (if you are aware of one, please let me know!). Thus CONFIG_CRASH_MAX_MEMORY_RANGES was 
> > born (rather borrowed from kexec-tools).
> >
> > So no dynamic computation is possible, yet.
> >
> > > > , but it all comes down to System RAM entries.
> > > >
> > > > I could perhaps offer an overly simplified example such that for 1GiB block
> > > > size, for example, the CRASH_MAX_MEMORY_RANGES of 32768 would allow for 32TiB
> > > > of memory?
> > >
> > > Yes, and stick it somewhere in Documentation/admin-guide/kdump/ and
> > > refer to it in that help text so that people can find it and read how to
> > > use your new option.
> > ok
> >
> > > > The kbuf.bufsz value is obtained via a call to prepare_elf_headers(); I can
> > > > not initialize it at its declaration.
> > >
> > > Sorry, I meant this:
> > >
> > > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > > index 8fc7d678ac72..ee6fd9f1b2b9 100644
> > > --- a/arch/x86/kernel/crash.c
> > > +++ b/arch/x86/kernel/crash.c
> > > @@ -395,8 +395,9 @@ int crash_load_segments(struct kimage *image)
> > >       if (ret)
> > >           return ret;
> > >   -    image->elf_headers = kbuf.buffer;
> > > -    image->elf_headers_sz = kbuf.bufsz;
> > > +    image->elf_headers    = kbuf.buffer;
> > > +    image->elf_headers_sz    = kbuf.bufsz;
> > > +    kbuf.memsz        = kbuf.bufsz;
> > >     #if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> > >       /* Ensure elfcorehdr segment large enough for hotplug changes */
> > > @@ -407,9 +408,8 @@ int crash_load_segments(struct kimage *image)
> > >       image->elf_headers_sz = kbuf.memsz;
> > >       image->elfcorehdr_index = image->nr_segments;
> > >       image->elfcorehdr_index_valid = true;
> > > -#else
> > > -    kbuf.memsz = kbuf.bufsz;
> > >   #endif
> > > +
> > >       kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
> > >       kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
> > >       ret = kexec_add_buffer(&kbuf);
> > ok
> >
> > > > I'm at a loss as to what to do differently here. You've raised this issue
> > > > before and I went back and looked at the suggestions then and I don't see
> > > > how that applies to this situation. How is this situation different than the
> > > > #ifdef CONFIG_KEXEC_FILE that immediately preceeds it?
> > >
> > > See the diff at the end. I'm not saying this is how you should do it
> > > but it should give you a better idea. The logic being, the functions
> > > in the .c file don't really need ifdeffery around them - you're adding
> > > 1-2 functions and crash.c is not that big - so they can be built in
> > > unconditionally. You'd need the ifdeffery *in the header only* when
> > > crash.c is not being built.
> > ok; I've overlooked that scenario.
> > > But I've done it with ifdeffery in the .c file now because yes, the
> > > kexec code is a minefield of ifdeffery. Hell, there's ifdeffery even in
> > > the headers for structs. Ifdeffery you don't really need. Someone should
> > > clean that up and simplify this immensely.
> >
> > ok
> >
> > > > Currently there is a concurrent effort for PPC support by Sourabh
> > > > Jain, and in that effort arch_map_crash_pages() is using __va(paddr).
> > >
> > > Why?
> > >
> > > > I do not know the nuances between kmap_local_page() and __va() to
> > > > answer the question.
> > >
> > > kmap_local_page() is a generic interface and it should work on any arch.
> > >
> > > And it is documented even:
> > >
> > > $ git grep kmap_local_page Documentation/
> > >
> > > > If kmap_local_page() works for all archs, then I'm happy to drop these
> > > > arch_ variants and use it directly.
> > >
> > > Yes, pls do.
> >
> > I'll check with Sourabh to see if PPC can work with kmap_local_page().
> I think kmap_local_page do support on  PowerPC. But can you explain why we need this
> function here, aren't the reserve memory already available to use?

On x86, attempts to access the elfcorehdr without mapping it did not work (resulted
in a fault).

Let me know if using kmap_local_page() in place of __va() in arch_map_crash_pages().
If it does, then I can eliminate arch_un/map_crash_pages() and use kmap_local_page()
directly.

Thanks,
eric
> Thanks,
> Sourabh Jain