[Crash-utility] increase __PHYSICAL_MASK_SHIFT_XEN?

[lijiang]

Hi, Jiri
在 2021年01月22日 01:00, crash-utility-request at redhat.com 写道:
> Date: Thu, 21 Jan 2021 17:18:29 +0100
> From: Jiri Bohac <jbohac at suse.cz>
> To: crash-utility at redhat.com
> Cc: anderson at redhat.com
> Subject: [Crash-utility] increase __PHYSICAL_MASK_SHIFT_XEN?
> Message-ID: <20210121161829.vizm27sb6pfrgmrl at dwarf.suse.cz>
> Content-Type: text/plain; charset=us-ascii
> 
> Hi,
> 
> I've just come across a situation where crash failed to open a
> dump generated by a 4.12 XEN PV dom0 kernel, terminating with
> this message:
> 
> crash: read error: physical address: ffffffffffffffff  type: "pud page"
> 
> The problem is a failed machine-to-physical translation.
> xen_m2p() returns an error (-1UL) and x86_64_pud_offset() than
> uses that value as a physical address.
> 
> I debugged the problem by running crash inside gdb. The backtrace was:
>  #0  xen_m2p (machine=973135872) at kernel.c:9714
>  #1  0x000000000053ed4a in x86_64_pud_offset (pgd_pte=3299508019303, vaddr=18446744072642223552, verbose=0, IS_XEN=1)
>      at x86_64.c:1889
>  #2  0x0000000000540c8a in x86_64_kvtop_xen_wpt (tc=0x0, kvaddr=18446744072642223552, paddr=0x7fffffffdb30, verbose=0)
>      at x86_64.c:2523
>  #3  0x00000000005407d0 in x86_64_kvtop (tc=0x0, kvaddr=18446744072642223552, paddr=0x7fffffffdb30, verbose=0)
>      at x86_64.c:2413
>  #4  0x0000000000491a97 in kvtop (tc=0x0, kvaddr=18446744072642223552, paddr=0x7fffffffdb30, verbose=0) at memory.c:3062
>  #5  0x000000000048f3f0 in readmem (addr=18446744072642223552, memtype=1, buffer=0xba63a0 <shared_bufs>, size=832,
>      type=0x92e1f2 "module struct", error_handle=6) at memory.c:2314
>  #6  0x00000000005071e2 in module_init () at kernel.c:3699
>  ....
> 
> I tracked the problem to a wrong value of
> __PHYSICAL_MASK_SHIFT_XEN. The current value of 40 does not
> correspond to the current kernel value of 52 since kernel commit
> 6f0e8bf16730a36ff6773802d8c8df56d10e60cd (xen: support 52 bit
> physical addresses in pv guests).
> 
> The result is visible in the above backtrace:
> x86_64_pud_offset() is called with pgd_pte=0x3003a00e067 and that
> value is wrongly masked by "pud_paddr = pgd_pte &
> PHYSICAL_PAGE_MASK" and passed to xen_m2p() as 0x3a00e000 instead
> of 0x3003a00e000, causing the m2p translation to fail.
> 

Thank you for sharing the details.

> Setting __PHYSICAL_MASK_SHIFT_XEN to 52 fixes the problem with
> this dump.
> 
> But I am not confident it's a safe change. My understanding is
> that it should be safe, as all the unused bits of the physical
> address inside the PTEs should be 0 and thus having the mask wider
> than necessary should not hurt. But I am suspicious if my
> reasoning is correct. Why does crash go into such trouble
> differentiating between different kernels and sets
> machdep->machspec->physical_mask_shift dynamically to one of
> __PHYSICAL_MASK_SHIFT_XEN (40), __PHYSICAL_MASK_SHIFT_2_6 (46),

The reason is that the supported kernel virtual address allows to
be configured on other architectures, and that still relies on the
hardware capability.

> and __PHYSICAL_MASK_SHIFT_5LEVEL (52)? Would something break if
> it were always set to 52? The commit adding the logic is
> 307e7f35.
> 
It's time to update the physical mask for xen, but can you help to
test the address translation via some command such as vtop, ptov,
etc? Or follow up the 5-level page changes to check if it has the
capability(cr4 & CR4_LA57)?

Thanks.
Lianbo

> Thanks,
> 
> -- Jiri Bohac <jbohac at suse.cz> SUSE Labs, Prague, Czechia