RE: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo

From: Kazuhito Hagio <k-hagio@ab.jp.nec.com>
To: James Morse <james.morse@arm.com>, Dave Young <dyoung@redhat.com>,
	"Bhupesh Sharma" <bhsharma@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	"lijiang@redhat.com" <lijiang@redhat.com>,
	"bhe@redhat.com" <bhe@redhat.com>,
	"ard.biesheuvel@linaro.org" <ard.biesheuvel@linaro.org>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	"kexec@lists.infradead.org" <kexec@lists.infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	"anderson@redhat.com" <anderson@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: RE: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo
Date: Wed, 13 Feb 2019 19:52:14 +0000	[thread overview]
Message-ID: <4AE2DC15AC0B8543882A74EA0D43DBEC03568504@BPXM09GP.gisp.nec.co.jp> (raw)
In-Reply-To: <bed51e6e-3dcb-8e07-1d31-9d81bfca20e5@arm.com>

On 2/13/2019 1:22 PM, James Morse wrote:
> Hi guys,
> 
> On 13/02/2019 11:15, Dave Young wrote:
> > On 02/12/19 at 11:03pm, Kazuhito Hagio wrote:
> >> On 2/12/2019 2:59 PM, Bhupesh Sharma wrote:
> >>> BTW, in the makedumpfile enablement patch thread for ARMv8.2 LVA
> >>> (which I sent out for 52-bit User space VA enablement) (see [0]), Kazu
> >>> mentioned that the changes look necessary.
> >>>
> >>> [0]. http://lists.infradead.org/pipermail/kexec/2019-February/022431.html
> >>
> >>>>> The increased 'PTRS_PER_PGD' value for such cases needs to be then
> >>>>> calculated as is done by the underlying kernel
> 
> Aha! Nothing to do with which-bits-are-pfn in the tables...
> 
> You need to know if the top level PGD is 512bytes or bigger. As we use a
> kmem-cache the adjacent data could be some else's page tables.
> 
> Is this really a problem though? You can't pull the user-space pgd pointers out
> of no-where, you must have walked some task_struct and struct_mm's to find them.
> In which case you would have the VMAs on hand to tell you if its in the mapped
> user range.
> 
> It would be good to avoid putting something arch-specific in here if we can at
> all help it.
> 
> 
> >>>>> (see
> >>>>> 'arch/arm64/include/asm/pgtable-hwdef.h' for details):
> >>>>>
> >>>>> #define PTRS_PER_PGD          (1 << (MAX_USER_VA_BITS - PGDIR_SHIFT))
> >>
> >> Yes, this is the reason why makedumpfile needs the MAX_USER_VA_BITS.
> >> It is used for pgd_index() also in makedumpfile to walk page tables.
> >>
> >> /* to find an entry in a page-table-directory */
> >> #define pgd_index(addr)         (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1))
> >
> > Since Dave mentioned crash tool does not need it, but crash should also
> > travel the pg tables.

The crash utility is always invoked with vmlinux, so it can read the
vabits_user variable directly from vmcore, but makedumpfile can not.

> > If this is really necessary it would be good to describe what will
> > happen without the patch, eg. some user visible error from an actual test etc.
> 
> Yes please, it would really help if there was a specific example we could discuss.

With 52-bit user space and 48-bit kernel space configuration,
makedumpfile will not be able to convert a virtual kernel address
to a physical address, and fail to capture a dumpfile, because the
pgd_index() will return a wrong index.

But I don't have any suitable test system on hand, so have not tried
the kernel configuration actually. If found, I'll try.

Bhupesh, do you have any test result?

Thanks,
Kazu

> 
> 
> Thanks,
> 
> James

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