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

[Sourabh Jain]

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?

Thanks,
Sourabh Jain