Unable to build LLDB on Amazon Linux 2 ARM64

[DoDoENT]

I'm trying to build full LLVM 13 toolchain within Amazon Linux 2 ARM64 docker and compilation of LLDB fails with:

/home/build/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.cpp:63:28: error: variable has incomplete type 'struct user_sve_header'
    struct user_sve_header sve_header;
                           ^
/home/build/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.cpp:63:12: note: forward declaration of 'user_sve_header'
    struct user_sve_header sve_header;
           ^
/home/build/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.cpp:376:13: error: use of undeclared identifier 'sve_vl_valid'
        if (sve_vl_valid(vg_value * 8)) {
            ^
/home/build/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.cpp:562:10: error: use of undeclared identifier 'sve_vl_valid'
    if (!sve_vl_valid(m_sve_header.vl)) {
         ^
/home/build/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.cpp:930:11: error: use of undeclared identifier 'sve_vl_valid'
      if (sve_vl_valid(m_sve_header.vl))
          ^
4 errors generated.

I've discovered that a similar issue has been also discovered in spack and the underlying problem is that Amazon Linux 2 is running on Kernel 4.14 which does not provide user_sve_header that has been added to kernel 4.15.

The main question is why does LLDB need a kernel with SVE support on non-SVE CPU? Should we add support for building the LLDB without SVE support?

[llvmbot]

@llvm/issue-subscribers-lldb

[DavidSpickett]

@omjavaid

[omjavaid]

LLDB build does not require kernel support for SVE but requires header to be present in your toolchain sysroot path. You may use a fairly recent arm toolchain like for example https://developer.arm.com/-/media/Files/downloads/gnu-a/10.3-2021.07/binrel/gcc-arm-10.3-2021.07-x86_64-aarch64-none-linux-gnu.tar.xz
It contains an updated ptrace.h containing user_sve_header definitions at libc/usr/include/asm/ptrace.h

[EugeneZelenko]

I think will be reasonable to check availability of user_sve_header definitions at CMake configuration time.

[nolongerwait]

So, how should we solve this problem?

[DavidSpickett]

The main question is why does LLDB need a kernel with SVE support on non-SVE CPU? Should we add support for building the LLDB without SVE support?

NativeRegisterContextLinux_arm64 - native meaning that this code is built into lldb-server.

We don't have a "single image" commitment like the Linux Kernel does for AArch64 but I think it would be pretty annoying if randomly some distro packages didn't support SVE because of the host they built on. Especially given that Amazon Linux may run on Gravitons that do in fact have SVE, though your kernel is presumably hiding that in this case.

I suggest you figure out a way to provide a definition of user_sve_header in LLDB itself, much like we do for new ptrace constants. I don't remember it being a complicated thing.

In fact we already have a copy in lldb/source/Plugins/Process/Utility/LinuxPTraceDefines_arm64sve.h. Maybe you can just include that in the native file, if that's going to cause a bunch of conflicts, duplicate the definition. We are not going to be changing it anyway, it has to match the kernel's type.