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Date:   Tue, 16 Aug 2022 10:23:15 -0500
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Subject: Re: [PATCH v10 8/8] x86/crash: Add x86 crash hotplug support
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To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, vgoyal@redhat.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, 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: <20220721181747.1640-1-eric.devolder@oracle.com>
 <20220721181747.1640-9-eric.devolder@oracle.com>
 <Yvbxgvk+5HqEl7J2@MiWiFi-R3L-srv>
From:   Eric DeVolder <eric.devolder@oracle.com>
In-Reply-To: <Yvbxgvk+5HqEl7J2@MiWiFi-R3L-srv>
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On 8/12/22 19:34, Baoquan He wrote:
> On 07/21/22 at 02:17pm, Eric DeVolder wrote:
> ...snip....
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index e58798f636d4..bb59596c8bea 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -2065,6 +2065,17 @@ 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
> 
> Do we need to enforce the value with page align and minimal size? I

Are you asking about the value CRASH_MAX_MEMORY_RANGES? This value represents
the maximum number of memory ranges, and there Elf64_Phdrs, that we need to
allow for elfcorehdr memory. So I'm not sure what the concern for alignment
is. I suppose we could also institute a minimum size for this value, say 1024.

> checked crash_load_segments() in arch/x86/kernel/crash.c, it does the
> page size aligning in kexec_add_buffer(). And in
> load_crashdump_segments() of
> kexec-tools/kexec/arch/i386/crashdump-x86.c, it creates elfcorehdr at
> below code, the align is 1024, and in generic add_buffer()
> implementation, it enforces the memsz page aligned, and changes the
> passed align as page alignment.
> 
> 
> 	elfcorehdr = add_buffer(info, tmp, bufsz, memsz, align, min_base,
>                                                          max_addr, -1);
> 
> Maybe we should at least mention this in the help text to notice people.

Unfortunately I do not yet understand the concern being raised.

> 
>> +	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.
>> +
>>   config KEXEC_JUMP
>>   	bool "kexec jump"
>>   	depends on KEXEC && HIBERNATION
>> diff --git a/arch/x86/include/asm/crash.h b/arch/x86/include/asm/crash.h
>> index 8b6bd63530dc..96051d8e4b45 100644
>> --- a/arch/x86/include/asm/crash.h
>> +++ b/arch/x86/include/asm/crash.h
>> @@ -9,4 +9,24 @@ int crash_setup_memmap_entries(struct kimage *image,
>>   		struct boot_params *params);
>>   void crash_smp_send_stop(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, unsigned int cpu);
>> +#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 /* _ASM_X86_CRASH_H */
>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>> index 9ceb93c176a6..55dda4fcde6e 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 +398,17 @@ 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);
> 
> Do we need to break the line to 80 chars?

Sure, I will do so.

> 
>> +	/* For marking as usable to crash kernel */
>> +	image->elf_headers_sz = kbuf.memsz;
> 
> Do we need this code comment?

Well, it did take me a while to figure this particular item out in order for all
this code to work right (else the crash kernel would fail at boot time). So I
think it best to keep this comment.

> 
>> +	/* Record the index of the elfcorehdr segment */
>> +	image->elfcorehdr_index = image->nr_segments;
> 
> And this place?

Not necessarily needed, but I've found it useful.

> 
>> +	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);
>> @@ -412,3 +423,107 @@ int crash_load_segments(struct kimage *image)
>>   	return ret;
>>   }
>>   #endif /* CONFIG_KEXEC_FILE */
>> +
>> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
>> +void *arch_map_crash_pages(unsigned long paddr, unsigned long size)
>> +{
>> +	/*
>> +	 * 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.
>> +	 */
> 
> Can we move the code comment above function interface?

Yes

> 
>> +	void *ptr = NULL;
>> +
>> +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
> 
> Do we need this code comment? On ARCH where proctionion is made, we
> surely need to the protect/unprotect.

I will remove this; I've mentioned this in handle_hotplug_event() where these
protect/unprotect functions are called.

> 
>> +	if (size > 0) {
>> +		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
>> +
>> +		ptr = kmap_local_page(page);
>> +	}
>> +
>> +	return ptr;
>> +}
>> +
>> +void arch_unmap_crash_pages(void **ptr)
>> +{
>> +	if (ptr) {
>> +		if (*ptr)
>> +			kunmap_local(*ptr);
>> +		*ptr = NULL;
>> +	}
>> +}
>> +
>> +/**
>> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
>> + * @image: the active struct kimage
>> + * @hp_action: the hot un/plug action being handled
>> + * @cpu: when KEXEC_CRASH_HP_ADD/REMOVE_CPU, the cpu affected
>> + *
>> + * To accurately reflect hot un/plug changes, the elfcorehdr (which
>> + * is passed to the crash kernel via the elfcorehdr= parameter)
>> + * must be updated with the new list of CPUs and memories. The new
>> + * elfcorehdr is prepared in a kernel buffer, and then it is
>> + * written on top of the existing/old elfcorehdr.
>> + *
>> + * For hotplug changes to elfcorehdr to work, two conditions are
>> + * needed:
>> + * First, the segment containing the elfcorehdr must be large enough
>> + * to permit a growing number of resources. See the
>> + * CONFIG_CRASH_MAX_MEMORY_RANGES description.
>> + * Second, purgatory must explicitly exclude the elfcorehdr from the
>> + * list of segments it checks (since the elfcorehdr changes and thus
>> + * would require an update to purgatory itself to update the digest).
> 
> Isn't this generic concept to crash hotplug? Should we move it out to
> some generic place?

Yes, so I will relocate this.

> 
>> + *
>> + */
>> +void arch_crash_handle_hotplug_event(struct kimage *image,
>> +	unsigned int hp_action, unsigned int cpu)
> 
> The passed in 'cpu' is not used at all, what is it added for? I didn't
> see explanation about it.

Well its not used for x86, but as I recall, Sourabh Jain needed it for the PowerPC handler.

> 
>> +{
>> +	struct kexec_segment *ksegment;
>> +	unsigned char *ptr = NULL;
>> +	unsigned long elfsz = 0;
>> +	void *elfbuf = NULL;
>> +	unsigned long mem, memsz;
>> +
>> +	/*
>> +	 * Elfcorehdr_index_valid checked in crash_core:handle_hotplug_event()
>> +	 */
>> +	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");
>> +		goto out;
>> +	}
>> +	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.
>> +	 * Copy new elfcorehdr into 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((void *)ptr, elfbuf, elfsz);
>> +		xchg(&kexec_crash_image, image);
>> +	}
>> +	arch_unmap_crash_pages((void **)&ptr);
>> +	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
>> +
>> +out:
>> +	if (elfbuf)
>> +		vfree(elfbuf);
>> +}
>> +#endif
>> -- 
>> 2.31.1
>>
>