linux-kernel - [PATCH v3 1/3] arm64, vmcoreinfo : Append 'PTRS_PER

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Date:   Wed, 20 Mar 2019 10:39:32 +0530
From:   Bhupesh Sharma <bhsharma@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     bhsharma@...hat.com, bhupesh.linux@...il.com,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        James Morse <james.morse@....com>,
        Dave Anderson <anderson@...hat.com>,
        Kazuhito Hagio <k-hagio@...jp.nec.com>,
        kexec@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org
Subject: [PATCH v3 1/3] arm64, vmcoreinfo : Append 'PTRS_PER_PGD' to vmcoreinfo

With ARMv8.2-LVA architecture extension availability, arm64 hardware
which supports this extension can support a virtual address-space upto
52-bits.

Since at the moment we enable the support of this extension in kernel
via CONFIG flags, e.g.
 - User-space 52-bit LVA via CONFIG_ARM64_USER_VA_BITS_52

so, there is no clear mechanism in the user-space right now to
determine these CONFIG flag values and hence determine the maximum
virtual address space supported by the underlying kernel.

User-space tools like 'makedumpfile' therefore are broken currently
as they have no proper method to calculate the 'PTRS_PER_PGD' value
which is required to perform a page table walk to determine the
physical address of a corresponding virtual address found in
kcore/vmcoreinfo.

If one appends 'PTRS_PER_PGD' number to vmcoreinfo for arm64,
it can be used in user-space to determine the maximum virtual address
supported by underlying kernel.

A reference 'makedumpfile' implementation which uses this approach to
determining the maximum physical address is available in [0].

[0]. https://github.com/bhupesh-sharma/makedumpfile/blob/52-bit-va-support-via-vmcore-upstream-v3/arch/arm64.c#L459

Cc: Mark Rutland <mark.rutland@....com>
Cc: Will Deacon <will.deacon@....com>
Cc: James Morse <james.morse@....com>
Cc: Dave Anderson <anderson@...hat.com>
Cc: Kazuhito Hagio <k-hagio@...jp.nec.com>
Cc: linux-kernel@...r.kernel.org
Cc: kexec@...ts.infradead.org
Cc: linux-arm-kernel@...ts.infradead.org
Suggested-by: Steve Capper <Steve.Capper@....com>
Signed-off-by: Bhupesh Sharma <bhsharma@...hat.com>
---
 arch/arm64/kernel/crash_core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kernel/crash_core.c b/arch/arm64/kernel/crash_core.c
index ca4c3e12d8c5..123a42c56b8e 100644
--- a/arch/arm64/kernel/crash_core.c
+++ b/arch/arm64/kernel/crash_core.c
@@ -10,6 +10,7 @@
 void arch_crash_save_vmcoreinfo(void)
 {
 	VMCOREINFO_NUMBER(VA_BITS);
+	VMCOREINFO_NUMBER(PTRS_PER_PGD);
 	/* Please note VMCOREINFO_NUMBER() uses "%d", not "%x" */
 	vmcoreinfo_append_str("NUMBER(kimage_voffset)=0x%llx\n",
 						kimage_voffset);
-- 
2.7.4