Hi Amit, On Fri, Feb 21, 2020 at 2:36 PM Amit Kachhap <amit.kach...@arm.com> wrote: > > 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.
Sorry for the delay. I was caught up in some other urgent arm64 user-space issues. I am preparing the v6 now and hopefully will be able to post it out for review later today. Thanks, Bhupesh _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec