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From:   Bhupesh Sharma <bhsharma@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     bhsharma@redhat.com, bhupesh.linux@gmail.com,
        Boris Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        James Morse <james.morse@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Dave Anderson <anderson@redhat.com>,
        Kazuhito Hagio <k-hagio@ab.jp.nec.com>, x86@kernel.org,
        linuxppc-dev@lists.ozlabs.org,
        linux-arm-kernel@lists.infradead.org, kexec@lists.infradead.org
Subject: [PATCH v2 2/2] crash_core, vmcoreinfo: Append 'MAX_PHYSMEM_BITS' to vmcoreinfo
Date:   Sun, 10 Mar 2019 15:34:02 +0530
Message-Id: <1552212242-9479-3-git-send-email-bhsharma@redhat.com>
In-Reply-To: <1552212242-9479-1-git-send-email-bhsharma@redhat.com>
References: <1552212242-9479-1-git-send-email-bhsharma@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Right now user-space tools like 'makedumpfile' and 'crash' need to rely
on a best-guess method of determining value of 'MAX_PHYSMEM_BITS'
supported by underlying kernel.

This value is used in user-space code to calculate the bit-space
required to store a section for SPARESMEM (similar to the existing
calculation method used in the kernel implementation):

  #define SECTIONS_SHIFT    (MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)

Now, regressions have been reported in user-space utilities
like 'makedumpfile' and 'crash' on arm64, with the recently added
kernel support for 52-bit physical address space, as there is
no clear method of determining this value in user-space
(other than reading kernel CONFIG flags).

As per suggestion from makedumpfile maintainer (Kazu), it makes more
sense to append 'MAX_PHYSMEM_BITS' to vmcoreinfo in the core code itself
rather than in arch-specific code, so that the user-space code for other
archs can also benefit from this addition to the vmcoreinfo and use it
as a standard way of determining 'SECTIONS_SHIFT' value in user-land.

A reference 'makedumpfile' implementation which reads the
'MAX_PHYSMEM_BITS' value from vmcoreinfo in a arch-independent fashion
is available here:

[0]. https://github.com/bhupesh-sharma/makedumpfile/blob/remove-max-phys-mem-bit-v1/arch/ppc64.c#L471

Cc: Boris Petkov <bp@alien8.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Dave Anderson <anderson@redhat.com>
Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.com>
Cc: x86@kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: kexec@lists.infradead.org
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
---
 kernel/crash_core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 093c9f917ed0..44b90368e183 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -467,6 +467,7 @@ static int __init crash_save_vmcoreinfo_init(void)
 #define PAGE_OFFLINE_MAPCOUNT_VALUE	(~PG_offline)
 	VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);
 #endif
+	VMCOREINFO_NUMBER(MAX_PHYSMEM_BITS);
 
 	arch_crash_save_vmcoreinfo();
 	update_vmcoreinfo_note();
-- 
2.7.4