Hi Bhupesh,
On 1/13/20 5:44 PM, Bhupesh Sharma wrote:
Hi James,
On 01/11/2020 12:30 AM, Dave Anderson wrote:
----- Original Message -----
Hi Bhupesh,
On 25/12/2019 19:01, Bhupesh Sharma wrote:
On 12/12/2019 04:02 PM, James Morse wrote:
On 29/11/2019 19:59, Bhupesh Sharma wrote:
vabits_actual variable on arm64 indicates the actual VA space size,
and allows a single binary to support both 48-bit and 52-bit VA
spaces.
If the ARMv8.2-LVA optional feature is present, and we are running
with a 64KB page size; then it is possible to use 52-bits of address
space for both userspace and kernel addresses. However, any kernel
binary that supports 52-bit must also be able to fall back to 48-bit
at early boot time if the hardware feature is not present.
Since TCR_EL1.T1SZ indicates the size offset of the memory region
addressed by TTBR1_EL1 (and hence can be used for determining the
vabits_actual value) it makes more sense to export the same in
vmcoreinfo rather than vabits_actual variable, as the name of the
variable can change in future kernel versions, but the architectural
constructs like TCR_EL1.T1SZ can be used better to indicate intended
specific fields to user-space.
User-space utilities like makedumpfile and crash-utility, need to
read/write this value from/to vmcoreinfo
(write?)
Yes, also write so that the vmcoreinfo from an (crashing) arm64
system can
be used for
analysis of the root-cause of panic/crash on say an x86_64 host using
utilities like
crash-utility/gdb.
I read this as as "User-space [...] needs to write to vmcoreinfo".
That's correct. But for writing to vmcore dump in the kdump kernel, we
need to read the symbols from the vmcoreinfo in the primary kernel.
for determining if a virtual address lies in the linear map range.
I think this is a fragile example. The debugger shouldn't need to know
this.
Well that the current user-space utility design, so I am not sure we
can
tweak that too much.
The user-space computation for determining whether an address lies in
the linear map range is the same as we have in kernel-space:
#define __is_lm_address(addr) (!(((u64)addr) &
BIT(vabits_actual -
1)))
This was changed with 14c127c957c1 ("arm64: mm: Flip kernel VA
space"). If
user-space
tools rely on 'knowing' the kernel memory layout, they must have to
constantly be fixed
and updated. This is a poor argument for adding this to something that
ends up as ABI.
See above. The user-space has to rely on some ABI/guaranteed
hardware-symbols which can be
used for 'determining' the kernel memory layout.
I disagree. Everything and anything in the kernel will change. The
ABI rules apply to
stuff exposed via syscalls and kernel filesystems. It does not apply
to kernel internals,
like the memory layout we used yesterday. 14c127c957c1 is a case in
point.
A debugger trying to rely on this sort of thing would have to play
catchup whenever it
changes.
Exactly. That's the whole point.
The crash utility and makedumpfile are not in the same league as other
user-space tools.
They have always had to "play catchup" precisely because they depend
upon kernel internals,
which constantly change.
I agree with you and DaveA here. Software user-space debuggers are
dependent on kernel internals (which can change from time-to-time) and
will have to play catch-up (which has been the case since the very start).
Unfortunately we don't have any clear ABI for software debugging tools -
may be something to look for in future.
A case in point is gdb/kgdb, which still needs to run with KASLR
turned-off (nokaslr) for debugging, as it confuses gdb which resolve
kernel symbol address from symbol table of vmlinux. But we can
work-around the same in makedumpfile/crash by reading the 'kaslr_offset'
value. And I have several users telling me now they cannot use gdb on
KASLR enabled kernel to debug panics, but can makedumpfile + crash
combination to achieve the same.
So, we should be looking to fix these utilities which are broken since
the 52-bit changes for arm64. Accordingly, I will try to send the v6
soon while incorporating the comments posted on the v5.
Any update on the next v6 version. Since this patch series is fixing the
current broken kdump so need this series to add some more fields in
vmcoreinfo for Pointer Authentication work.
Thanks,
Amit Daniel
Thanks,
Bhupesh
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