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Date:   Mon, 26 Sep 2022 14:19:38 -0500
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Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support
Content-Language: en-US
From:   Eric DeVolder <eric.devolder@oracle.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com,
        tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com,
        hpa@zytor.com, nramas@linux.microsoft.com, thomas.lendacky@amd.com,
        robh@kernel.org, efault@gmx.de, rppt@kernel.org, david@redhat.com,
        sourabhjain@linux.ibm.com, konrad.wilk@oracle.com,
        boris.ostrovsky@oracle.com
References: <20220909210509.6286-1-eric.devolder@oracle.com>
 <20220909210509.6286-8-eric.devolder@oracle.com>
 <Yx7XEcXZ8PwwQW95@nazgul.tnic>
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Boris,
I've a few questions for you below. With your responses, I am hopeful we can finish this series soon!
Thanks,
eric

On 9/13/22 14:12, Eric DeVolder wrote:
> Boris,
> Thanks for the feedback! Inline responses below.
> eric
> 
> On 9/12/22 01:52, Borislav Petkov wrote:
>> On Fri, Sep 09, 2022 at 05:05:09PM -0400, Eric DeVolder wrote:
>>> For x86_64, when CPU or memory is hot un/plugged, the crash
>>> elfcorehdr, which describes the CPUs and memory in the system,
>>> must also be updated.
>>>
>>> When loading the crash kernel via kexec_load or kexec_file_load,
>>
>> Please end function names with parentheses. Check the whole patch pls.
> Done.
> 
>>
>>> the elfcorehdr is identified at run time in
>>> crash_core:handle_hotplug_event().
>>>
>>> To update the elfcorehdr for x86_64, a new elfcorehdr must be
>>> generated from the available CPUs and memory. The new elfcorehdr
>>> is prepared into a buffer, and then installed over the top of
>>> the existing elfcorehdr.
>>>
>>> In the patch 'kexec: exclude elfcorehdr from the segment digest'
>>> the need to update purgatory due to the change in elfcorehdr was
>>> eliminated.  As a result, no changes to purgatory or boot_params
>>> (as the elfcorehdr= kernel command line parameter pointer
>>> remains unchanged and correct) are needed, just elfcorehdr.
>>>
>>> To accommodate a growing number of resources via hotplug, the
>>> elfcorehdr segment must be sufficiently large enough to accommodate
>>> changes, see the CRASH_MAX_MEMORY_RANGES configure item.
>>>
>>> With this change, crash hotplug for kexec_file_load syscall
>>> is supported.
>>
>> Redundant sentence.
> Removed.
> 
>>
>>> The kexec_load is also supported, but also
>>> requires a corresponding change to userspace kexec-tools.
>>
>> Ditto.
> Removed.
> 
>>
>>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>>> Acked-by: Baoquan He <bhe@redhat.com>
>>> ---
>>>   arch/x86/Kconfig             |  11 ++++
>>>   arch/x86/include/asm/kexec.h |  20 +++++++
>>>   arch/x86/kernel/crash.c      | 102 +++++++++++++++++++++++++++++++++++
>>>   3 files changed, 133 insertions(+)
>>>
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index f9920f1341c8..cdfc9b2fdf98 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -2056,6 +2056,17 @@ config CRASH_DUMP
>>>         (CONFIG_RELOCATABLE=y).
>>>         For more details see Documentation/admin-guide/kdump/kdump.rst
>>> +config CRASH_MAX_MEMORY_RANGES
>>> +    depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU || MEMORY_HOTPLUG)
>>> +    int
>>> +    default 32768
>>> +    help
>>> +      For the kexec_file_load path, specify the maximum number of
>>> +      memory regions, eg. as represented by the 'System RAM' entries
>>> +      in /proc/iomem, that the elfcorehdr buffer/segment can accommodate.
>>> +      This value is combined with NR_CPUS and multiplied by Elf64_Phdr
>>> +      size to determine the final buffer size.
>>
>> If I'm purely a user, I'm left wondering how to determine what to
>> specify. Do you have a guidance text somewhere you can point to from
>> here?
> 
> This topic was discussed previously https://lkml.org/lkml/2022/3/3/372.
> 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, 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?
> 
>>
>>> +
>>>   config KEXEC_JUMP
>>>       bool "kexec jump"
>>>       depends on KEXEC && HIBERNATION
>>> diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
>>> index a3760ca796aa..432073385b2d 100644
>>> --- a/arch/x86/include/asm/kexec.h
>>> +++ b/arch/x86/include/asm/kexec.h
>>> @@ -212,6 +212,26 @@ typedef void crash_vmclear_fn(void);
>>>   extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
>>>   extern void kdump_nmi_shootdown_cpus(void);
>>> +void *arch_map_crash_pages(unsigned long paddr, unsigned long size);
>>> +#define arch_map_crash_pages arch_map_crash_pages
>>> +
>>> +void arch_unmap_crash_pages(void **ptr);
>>> +#define arch_unmap_crash_pages arch_unmap_crash_pages
>>> +
>>> +void arch_crash_handle_hotplug_event(struct kimage *image,
>>> +        unsigned int hp_action);
>>> +#define arch_crash_handle_hotplug_event arch_crash_handle_hotplug_event
>>> +
>>> +#ifdef CONFIG_HOTPLUG_CPU
>>> +static inline int crash_hotplug_cpu_support(void) { return 1; }
>>> +#define crash_hotplug_cpu_support crash_hotplug_cpu_support
>>> +#endif
>>> +
>>> +#ifdef CONFIG_MEMORY_HOTPLUG
>>> +static inline int crash_hotplug_memory_support(void) { return 1; }
>>> +#define crash_hotplug_memory_support crash_hotplug_memory_support
>>> +#endif
>>> +
>>>   #endif /* __ASSEMBLY__ */
>>>   #endif /* _ASM_X86_KEXEC_H */
>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>> index 9ceb93c176a6..8fc7d678ac72 100644
>>> --- a/arch/x86/kernel/crash.c
>>> +++ b/arch/x86/kernel/crash.c
>>> @@ -25,6 +25,7 @@
>>>   #include <linux/slab.h>
>>>   #include <linux/vmalloc.h>
>>>   #include <linux/memblock.h>
>>> +#include <linux/highmem.h>
>>>   #include <asm/processor.h>
>>>   #include <asm/hardirq.h>
>>> @@ -397,7 +398,18 @@ int crash_load_segments(struct kimage *image)
>>>       image->elf_headers = kbuf.buffer;
>>>       image->elf_headers_sz = kbuf.bufsz;
>>> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
>>> +    /* Ensure elfcorehdr segment large enough for hotplug changes */
>>> +    kbuf.memsz =
>>> +        (CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES) *
>>> +            sizeof(Elf64_Phdr);
>>
>>
>>     kbuf.memsz  = CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES;
>>     kbuf.memsz *= sizeof(Elf64_Phdr);
>>
>> looks more readable to me.
> Done.
> 
>>
>>
>>> +    /* Mark as usable to crash kernel, else crash kernel fails on boot */
>>> +    image->elf_headers_sz = kbuf.memsz;
>>> +    image->elfcorehdr_index = image->nr_segments;
>>> +    image->elfcorehdr_index_valid = true;
>>> +#else
>>>       kbuf.memsz = kbuf.bufsz;
>>
>> Do that initialization at the top where you declare kbuf and get rid of
>> the #else branch.
> The kbuf.bufsz value is obtained via a call to prepare_elf_headers(); I can not initialize it at its 
> declaration.
> 
>>
>>> +#endif
>>>       kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
>>>       kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
>>>       ret = kexec_add_buffer(&kbuf);
>>> @@ -412,3 +424,93 @@ int crash_load_segments(struct kimage *image)
>>>       return ret;
>>>   }
>>>   #endif /* CONFIG_KEXEC_FILE */
>>> +
>>> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
>>
>> This ugly ifdeffery is still here. Why don't you have stubs for the
>> !defined() cases in the header so that you can drop those here?
>>
> 
> 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?
> 
> I've included a copy of the current state of this section below for additional markup.
> 
>>> +/*
>>> + * NOTE: The addresses and sizes passed to this routine have
>>> + * already been fully aligned on page boundaries. There is no
>>> + * need for massaging the address or size.
>>> + */
>>> +void *arch_map_crash_pages(unsigned long paddr, unsigned long size)
>>> +{
>>> +    void *ptr = NULL;
>>> +
>>> +    if (size > 0) {
>>> +        struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
>>> +
>>> +        ptr = kmap_local_page(page);
>>> +    }
>>> +
>>> +    return ptr;
>>> +}
>>
>>     if (size > 0)
>>         return kmap_local_page(pfn_to_page(paddr >> PAGE_SHIFT));
>>     else
>>         return NULL;
>>
>> That's it.
> Done.
> 
>>
>>> +
>>> +void arch_unmap_crash_pages(void **ptr)
>>> +{
>>> +    if (ptr) {
>>> +        if (*ptr)
>>> +            kunmap_local(*ptr);
>>> +        *ptr = NULL;
>>> +    }
>>
>> Oh wow, this is just nuts. Why does it have to pass in a pointer to
>> pointer which you have to carefully check twice? And why is it a void
>> **?
> A long time ago this made sense, but it no longer makes sense. I've corrected this.
> 
>>
>> And why are those called arch_ if all I see is the x86 variants? Are
>> there gonna be other arches? And even if, why can't the other arches do
>> kmap_local_page() too?
> Currently there is a concurrent effort for PPC support by Sourabh Jain, and in that effort 
> arch_map_crash_pages() is using __va(paddr).
> 
> I do not know the nuances between kmap_local_page() and __va() to answer the question.
> 
> If kmap_local_page() works for all archs, then I'm happy to drop these arch_ variants
> and use it directly.
> 
>>
>>> +}
>>> +
>>> +/**
>>> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
>>> + * @image: the active struct kimage
>>> + * @hp_action: the hot un/plug action being handled
>>> + *
>>> + * To accurately reflect hot un/plug changes, the new elfcorehdr
>>> + * is prepared in a kernel buffer, and then it is written on top
>>> + * of the existing/old elfcorehdr.
>>> + */
>>> +void arch_crash_handle_hotplug_event(struct kimage *image,
>>> +    unsigned int hp_action)
>>
>> Align arguments on the opening brace.
> Done.
> 
>>
>>> +{
>>> +    struct kexec_segment *ksegment;
>>> +    unsigned char *ptr = NULL;
>>> +    unsigned long elfsz = 0;
>>> +    void *elfbuf = NULL;
>>> +    unsigned long mem, memsz;
>>
>> Please sort function local variables declaration in a reverse christmas
>> tree order:
>>
>>     <type A> longest_variable_name;
>>     <type B> shorter_var_name;
>>     <type C> even_shorter;
>>     <type D> i;
>>
> Done.
> 
>>> +
>>> +    /*
>>> +     * Elfcorehdr_index_valid checked in crash_core:handle_hotplug_event()
>>
>> Elfcorehdr_index_valid??
> Comment reworked.
> 
> 
>>
>>> +     */
>>> +    ksegment = &image->segment[image->elfcorehdr_index];
>>> +    mem = ksegment->mem;
>>> +    memsz = ksegment->memsz;
>>> +
>>> +    /*
>>> +     * Create the new elfcorehdr reflecting the changes to CPU and/or
>>> +     * memory resources.
>>> +     */
>>> +    if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
>>> +        pr_err("crash hp: unable to prepare elfcore headers");
>>             ^^^^^^^^
>>
>> this thing is done with pr_fmt(). Grep the tree for examples.
> Done, thanks for pointing that out.
> 
>>
>>> +        goto out;
>>> +    }
>>
>> The three lines above reading ksegment need to be here, where the test
>> is done.
> Done.
> 
>>
>>> +    if (elfsz > memsz) {
>>> +        pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
>>> +            elfsz, memsz);
>>> +        goto out;
>>> +    }
>>> +
>>> +    /*
>>> +     * At this point, we are all but assured of success.
>>
>> Who is "we"?
>>
> Comment reworked.
> 
> 
> Here is a copy of the current state of this code, for determining how to address the question above.
> 
> #if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> 
> #undef pr_fmt
> #define pr_fmt(fmt) "crash hp: " fmt
> 
> /*
>   * NOTE: The addresses and sizes passed to this routine have
>   * already been fully aligned on page boundaries. There is no
>   * need for massaging the address or size.
>   */
> void *arch_map_crash_pages(unsigned long paddr, unsigned long size)
> {
>          if (size > 0)
>                  return kmap_local_page(pfn_to_page(paddr >> PAGE_SHIFT));
>          else
>                  return NULL;
> }
> 
> void arch_unmap_crash_pages(void *ptr)
> {
>          if (ptr)
>                  kunmap_local(ptr);
> }
> 
> /**
>   * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
>   * @image: the active struct kimage
>   * @hp_action: the hot un/plug action being handled
>   *
>   * To accurately reflect hot un/plug changes, the new elfcorehdr
>   * is prepared in a kernel buffer, and then it is written on top
>   * of the existing/old elfcorehdr.
>   */
> void arch_crash_handle_hotplug_event(struct kimage *image,
>                                      unsigned int hp_action)
> {
>          unsigned long mem, memsz;
>          unsigned long elfsz = 0;
>          void *elfbuf = NULL;
>          void *ptr;
> 
>          /*
>           * Create the new elfcorehdr reflecting the changes to CPU and/or
>           * memory resources.
>           */
>          if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
>                  pr_err("unable to prepare elfcore headers");
>                  goto out;
>          }
> 
>          /*
>           * Obtain address and size of the elfcorehdr segment, and
>           * check it against the new elfcorehdr buffer.
>           */
>          mem = image->segment[image->elfcorehdr_index].mem;
>          memsz = image->segment[image->elfcorehdr_index].memsz;
>          if (elfsz > memsz) {
>                  pr_err("update elfcorehdr elfsz %lu > memsz %lu",
>                          elfsz, memsz);
>                  goto out;
>          }
> 
>          /*
>           * Copy new elfcorehdr over the old elfcorehdr at destination.
>           */
>          ptr = arch_map_crash_pages(mem, memsz);
>          if (ptr) {
>                  /*
>                   * Temporarily invalidate the crash image while the
>                   * elfcorehdr is updated.
>                   */
>                  xchg(&kexec_crash_image, NULL);
>                  memcpy_flushcache(ptr, elfbuf, elfsz);
>                  xchg(&kexec_crash_image, image);
>          }
>          arch_unmap_crash_pages(ptr);
>          pr_debug("re-loaded elfcorehdr at 0x%lx\n", mem);
> 
> out:
>          if (elfbuf)
>                  vfree(elfbuf);
> }
> #endif
>