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

[Bhupesh Sharma <bhsharma@redhat.com>]

On Tue, Feb 12, 2019 at 4:14 PM Dave Young <dyoung@redhat.com> wrote:
>
> On 02/12/19 at 10:37am, Bhupesh Sharma wrote:
> > Hi Kazu,
> >
> > On 02/04/2019 09:34 PM, Kazuhito Hagio wrote:
> > > On 1/30/2019 8:48 PM, Dave Young wrote:
> > > > + more people
> > > > On 01/30/19 at 05:53pm, Bhupesh Sharma wrote:
> > > > > With ARMv8.2-LVA and LPA architecture extensions, arm64 hardware which
> > > > > supports these extensions can support upto 52-bit virtual and 52-bit
> > > > > physical addresses respectively.
> > > > >
> > > > > Since at the moment we enable the support of these extensions via CONFIG
> > > > > flags, e.g.
> > > > >   - LPA via CONFIG_ARM64_PA_BITS_52
> > > > >
> > > > > there are no clear mechanisms in user-space right now to
> > > > > deteremine these CONFIG flag values and also determine the PARange and
> > > > > VARange address values.
> > > > >
> > > > > User-space tools like 'makedumpfile' and 'crash-utility' can instead
> > > > > use the 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' values to determine
> > > > > the maximum virtual address and physical address (respectively)
> > > > > 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-pa-support-via-vmcore-v1/arch/arm64.c#L490
> > > >
> > > > I'm not objecting the patch, just want to make sure to make clear about
> > > > things and make sure these issues are aware by people, and leave arm
> > > > people to review the arm bits.
> > > >
> > > > 1. MAX_PHYSMEM_BITS
> > > > As we previously found, back to 2014 makedumpfile took a patch to read the
> > > > value from vmcore but the kernel patch was not accepted.
> > > > So we should first make clear if this is really needed, why other arches
> > > > do not need this in makedumpfile.
> > > >
> > > > If we really need it then should it be arm64 only?
> > > >
> > > > If it is arm64 only then the makedumpfile code should read this number
> > > > only for arm64.
> > >
> > > Sorry for the delay.
> > >
> > > According to the kernel patch, some of arm32 platforms may need it
> > > http://lists.infradead.org/pipermail/kexec/2014-May/011909.html
> > > but except for them (and arm64), makedumpfile can manage with kernel
> > > version and some switches to determine this value so far.
> > >
> > > >
> > > > Also Lianbo added the vmcoreinfo documents, I believe it stays in -tip
> > > > tree,  need to make sure to document this as well.
> > > >
> > > > 2. MAX_USER_VA_BITS
> > > > Does makedumpfile care about userspace VA bits?  I do not see other code
> > > > doing this,  Kazu and Dave A should be able to comment.
> > >
> > > The mapping makedumpfile uses on arm64 is swapper_pg_dir only, so
> > > unless the config affects its structure or something, makedumpfile
> > > will not need this value.
> >
> > I captured this case in more details while sending out the makedumpfile
> > enablement patch for ARMv8.2-LVA (see [0]), but here is a brief summary on
> > the same:
> >
> > Since at the moment we enable the support of the ARMv8.2-LVA extension for
> > 52-bit user-space VA in the kernel via a CONFIG flags
> > (CONFIG_ARM64_USER_VA_BITS_52), so there are no clear mechanisms in
> > user-space to determine this CONFIG
> > flag value and use it to determine the address range values.
> >
> > Since 'VA_BITS' are already exported via vmcoreinfo, if we export
> > 'MAX_USER_VA_BITS' as well, we can use the same in user-space to check if
> > the 'MAX_USER_VA_BITS' value is greater than 'VA_BITS'. If yes, then we are
> > running a use-case where user-space is 52-bit while the underlying kernel is
> > still 48-bit.
>
> Problem is why this is needed, it sounds like you are talking about some
> non-exist use case.

I already explained this in an earlier reply to your initial comments
(see <http://lists.infradead.org/pipermail/kexec/2019-January/022395.html>).
Perhaps you missed reading it, so here are the architecturally
possible and kernel supported uses cases with ARMv8.2 extensions
(depending on the combination of CONFIG flags and kernel version):

- 48-bit kernel VA + 48-bit user-space VA + 52-bit PA
- 48-bit kernel VA + 52-bit user-space VA + 52-bit PA
- 52-bit kernel VA + 52-bit user-space VA + 52-bit PA

Please see 'config ARM64_USER_VA_BITS_52'  help text inside
'arch/arm64/Kconfig' for more details:
config ARM64_USER_VA_BITS_52
    bool "52-bit (user)"
    depends on ARM64_64K_PAGES && (ARM64_PAN || !ARM64_SW_TTBR0_PAN)
    help
      Enable 52-bit virtual addressing for userspace when explicitly
      requested via a hint to mmap(). The kernel will continue to
      use 48-bit virtual addresses for its own mappings.

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

Thanks,
Bhupesh

> >
> > The increased 'PTRS_PER_PGD' value for such cases needs to be then
> > calculated as is done by the underlying kernel (see
> > 'arch/arm64/include/asm/pgtable-hwdef.h' for details):
> >
> > #define PTRS_PER_PGD          (1 << (MAX_USER_VA_BITS - PGDIR_SHIFT))
> >
> > Also, note that 'arch/arm64/include/asm/memory.h' defines 'MAX_USER_VA_BITS'
> > as 'VA_BITS' in case 'CONFIG_ARM64_USER_VA_BITS_52' is set to 'n':
> >
> > #ifdef CONFIG_ARM64_USER_VA_BITS_52
> > #define MAX_USER_VA_BITS      52
> > #else
> > #define MAX_USER_VA_BITS      VA_BITS
> > #endif
> >
> > So, makedumpfile will need this symbol exported in vmcore to make the above
> > determination.
> >
> > [0]. http://lists.infradead.org/pipermail/kexec/2019-February/022425.html
> >
> > Thanks,
> > Bhupesh
>
> Thanks
> Dave

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