linux-kernel - Re: [PATCH v3 09/12] ppc64/kexec_file: setup backup region for kdump kernel

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Date:   Wed, 15 Jul 2020 22:38:01 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Hari Bathini <hbathini@...ux.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel test robot <lkp@...el.com>,
        Pingfan Liu <piliu@...hat.com>,
        Kexec-ml <kexec@...ts.infradead.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Petr Tesarik <ptesarik@...e.cz>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Dave Young <dyoung@...hat.com>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v3 09/12] ppc64/kexec_file: setup backup region for kdump kernel


Hari Bathini <hbathini@...ux.ibm.com> writes:

> @@ -968,7 +1040,7 @@ int setup_new_fdt_ppc64(const struct kimage *image, void *fdt,
>
>  	/*
>  	 * Restrict memory usage for kdump kernel by setting up
> -	 * usable memory ranges.
> +	 * usable memory ranges and memory reserve map.
>  	 */
>  	if (image->type == KEXEC_TYPE_CRASH) {
>  		ret = get_usable_memory_ranges(&umem);
> @@ -980,6 +1052,24 @@ int setup_new_fdt_ppc64(const struct kimage *image, void *fdt,
>  			pr_err("Error setting up usable-memory property for kdump kernel\n");
>  			goto out;
>  		}
> +
> +		ret = fdt_add_mem_rsv(fdt, BACKUP_SRC_START + BACKUP_SRC_SIZE,
> +				      crashk_res.start - BACKUP_SRC_SIZE);

I believe this answers my question from the other email about how the
crashkernel is prevented from stomping in the crashed kernel's memory,
right? I needed to think for a bit to understand what the above
reservation was protecting. I think it's worth adding a comment.

> +		if (ret) {
> +			pr_err("Error reserving crash memory: %s\n",
> +			       fdt_strerror(ret));
> +			goto out;
> +		}
> +	}
> +
> +	if (image->arch.backup_start) {
> +		ret = fdt_add_mem_rsv(fdt, image->arch.backup_start,
> +				      BACKUP_SRC_SIZE);
> +		if (ret) {
> +			pr_err("Error reserving memory for backup: %s\n",
> +			       fdt_strerror(ret));
> +			goto out;
> +		}
>  	}

This is only true for KEXEC_TYPE_CRASH, if I'm following the code
correctly. I think it would be clearer to put the if above inside the if
for KEXEC_TYPE_CRASH to make it clearer.

>
>  	ret = setup_new_fdt(image, fdt, initrd_load_addr, initrd_len,

<snip>

> diff --git a/arch/powerpc/purgatory/purgatory_64.c b/arch/powerpc/purgatory/purgatory_64.c
> new file mode 100644
> index 0000000..1eca74c
> --- /dev/null
> +++ b/arch/powerpc/purgatory/purgatory_64.c
> @@ -0,0 +1,36 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * purgatory: Runs between two kernels
> + *
> + * Copyright 2020, Hari Bathini, IBM Corporation.
> + */
> +
> +#include <asm/purgatory.h>
> +#include <asm/crashdump-ppc64.h>
> +
> +extern unsigned long backup_start;
> +
> +static void *__memcpy(void *dest, const void *src, unsigned long n)
> +{
> +	unsigned long i;
> +	unsigned char *d;
> +	const unsigned char *s;
> +
> +	d = dest;
> +	s = src;
> +	for (i = 0; i < n; i++)
> +		d[i] = s[i];
> +
> +	return dest;
> +}
> +
> +void purgatory(void)
> +{
> +	void *dest, *src;
> +
> +	src = (void *)BACKUP_SRC_START;
> +	if (backup_start) {
> +		dest = (void *)backup_start;
> +		__memcpy(dest, src, BACKUP_SRC_SIZE);
> +	}
> +}

In general I'm in favor of using C code over assembly, but having to
bring in that relocation support just for the above makes me wonder if
it's worth it in this case.

--
Thiago Jung Bauermann
IBM Linux Technology Center