RE: [PATCH v3 1/3] arm64, vmcoreinfo : Append 'PTRS_PER_PGD' to vmcoreinfo

[Kazuhito Hagio <k-hagio@ab.jp.nec.com>]

Hi James,

Thank you for your comment.

-----Original Message-----
> Hi Kazu,
> 
> On 27/03/2019 16:07, Kazuhito Hagio wrote:
> > On 3/26/2019 12:36 PM, James Morse wrote:
> >> On 20/03/2019 05:09, Bhupesh Sharma wrote:
> >>> 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.
> >>
> >> I don't think this really solves the problem, it feels fragile.
> >>
> >> I can see how vmcoreinfo tells you VA_BITS==48, PAGE_SIZE==64K and PTRS_PER_PGD=1024.
> >> You can use this to work out that the top level page table size isn't consistent with a
> >> 48bit VA, so 52bit VA must be in use...
> >>
> >> But wasn't your problem walking the kernel page tables? In particular the offset that we
> >> apply because the tables were based on a 48bit VA shifted up in swapper_pg_dir.
> >>
> >> Where does the TTBR1_EL1 offset come from with this property? I assume makedumpfile
> >> hard-codes it when it sees 52bit is in use ... somewhere.
> 
> > My understanding is that the TTBR1_EL1 offset comes from a kernel
> > virtual address with the exported PTRS_PER_PGD.
> >
> > With T1SZ is 48bit and T0SZ is 52bit,
> 
> (PTRS_PER_PGD doesn't tell you this, PTRS_PER_PGD lets you spot something odd is
> happening, and this just happens to be the only odd combination today.)

I didn't intend to guess other things from PTRS_PER_PGD.

> > kva = 0xffff000000000000    <--- start of kernel virtual address
> 
> Does makedumpfile have this value? If the kernel were using 52bit VA for TTBR1 this value
> would be different.

This was an example address to show that pgd_index() automatically returns
a value including the offset for any kernel virtual address by the exported
PTRS_PER_PGD. In this case, even for the first virtual address, it returns
the non-zero value, which is the offset. (sorry for the poor explanation..)

So makedumpfile doesn't need the start address specifically to walk the page
tables, and I was thinking that exporting PTRS_PER_PGD may be stable unless
pgd_index() doesn't change.

> > pgd_index(kva) = (kva >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1)
> >                = (0xffff000000000000 >> 42) & (1024 - 1)
> >                = 0x00000000003fffc0 & 0x3ff
> >                = 0x3c0      <--- the offset (0x3c0) is included
> >
> > This is what kernel does now, so makedumpfile also wants to do.
> 
> Sure, and it would work today. I'm worried about tomorrow, where we support something new,
> and need to bundle new information out through vmcoreinfo. This ends up being used to
> fingerprint the kernel support, instead of as the value it was intended to be.

Yes, more stable and reasonable way is preferable.

> >> We haven't solved the problem!
> >>
> >> Today __cpu_setup() sets T0SZ and T1SZ differently for 52bit VA, but in the future it
> >> could set them the same, or different the other-way-round.
> >>
> >> Will makedumpfile using this value keep working once T1SZ is 52bit VA too? In this case
> >> there would be no ttbr offset.
> >
> > If T1SZ is 52bit, probably kernel virtual address starts from 0xfff0000000000000,
> 
> I didn't think this 'bottom of the ttbr1 mapping range' value was exposed anywhere.
> Where can user-space get this from? (I can't see it in the vmcoreinfo list)
> 
> 
> > then the offset becomes 0 with the pgd_index() above.
> > I think makedumpfile will keep working with that.
> 
> 
> Steve mentions a 52/48 combination in his kernel series:
> https://lore.kernel.org/linux-arm-kernel/20190218170245.14915-1-steve.capper@arm.com/
> 
> 
> I think vmcoreinfo-users will eventually need to spot 52bit used in TTBR1 and/or TTBR0,
> and possibly: configured, but not enabled in either. (this is because the bits are also
> used for pointer-auth, the kernel may be built for both pointer-auth and 52-bit VA, and
> chose which to enabled at boot based on some policy)
> 
> I don't see how you can do this with one value.
> I'd like to get this right now, so we user-space doesn't need updating again!

I need to understand the 52-bit kernel VA implementation, pointer-auth,
and Bhupesh's discussion. I will catch up them.

Thanks a lot!
Kazu

> 
> 
> Thanks,
> 
> James

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