mbox series

[0/2] m68k: Add vDSO support for TLS and clock_gettime64

Message ID 20260108061822.3896153-1-stefan.reinauer@coreboot.org (mailing list archive)
Headers
Series m68k: Add vDSO support for TLS and clock_gettime64 |

Message

Stefan Reinauer Jan. 8, 2026, 6:18 a.m. UTC 
  This series adds vDSO support for m68k, enabling userspace to access
the thread pointer and clock_gettime64 without syscall overhead.

The m68k vDSO provides a data page mapped at a fixed offset from the
ELF header (passed via AT_SYSINFO_EHDR).  The kernel maintains the
thread pointer and timing data in this page, updating them on context
switches.

Patch 1 optimizes __m68k_read_tp() to read the thread pointer directly
from the vDSO data page instead of using the get_thread_area syscall.
This significantly reduces overhead for TLS access, which is a frequent
operation in threaded applications.

Patch 2 enables the vDSO clock_gettime64 by defining
HAVE_CLOCK_GETTIME64_VSYSCALL along with the necessary VDSO_NAME and
VDSO_HASH for symbol lookup.  This allows glibc to use the
kernel-provided __vdso_clock_gettime64 for fast time retrieval.

Both changes include fallback paths for older kernels that lack vDSO
support.

Performance Results (QEMU Quadra 800)
-------------------------------------
Before (syscall):     ~2057 ns/call
After (vDSO):         ~675 ns/call
Improvement:          ~3x faster

Tested on qemu-system-m68k with Linux 6.18 (Debian/m68k).

Stefan Reinauer (2):
  m68k: Use vDSO for thread pointer access
  m68k: Enable vDSO clock_gettime64

 sysdeps/unix/sysv/linux/m68k/m68k-helpers.c | 77 ++++++++++++++++++++-
 sysdeps/unix/sysv/linux/m68k/sysdep.h       |  7 ++
 2 files changed, 83 insertions(+), 1 deletion(-)

Comments

Andreas K. Huettel Jan. 8, 2026, 9:46 p.m. UTC | #1 
Am Donnerstag, 8. Januar 2026, 07:18:20 Mitteleuropäische Normalzeit schrieb Stefan:
> This series adds vDSO support for m68k, enabling userspace to access
> the thread pointer and clock_gettime64 without syscall overhead.

After the release only please (given other concerns are addressed)

> 
> The m68k vDSO provides a data page mapped at a fixed offset from the
> ELF header (passed via AT_SYSINFO_EHDR).  The kernel maintains the
> thread pointer and timing data in this page, updating them on context
> switches.
> 
> Patch 1 optimizes __m68k_read_tp() to read the thread pointer directly
> from the vDSO data page instead of using the get_thread_area syscall.
> This significantly reduces overhead for TLS access, which is a frequent
> operation in threaded applications.
> 
> Patch 2 enables the vDSO clock_gettime64 by defining
> HAVE_CLOCK_GETTIME64_VSYSCALL along with the necessary VDSO_NAME and
> VDSO_HASH for symbol lookup.  This allows glibc to use the
> kernel-provided __vdso_clock_gettime64 for fast time retrieval.
> 
> Both changes include fallback paths for older kernels that lack vDSO
> support.
> 
> Performance Results (QEMU Quadra 800)
> -------------------------------------
> Before (syscall):     ~2057 ns/call
> After (vDSO):         ~675 ns/call
> Improvement:          ~3x faster
> 
> Tested on qemu-system-m68k with Linux 6.18 (Debian/m68k).
> 
> Stefan Reinauer (2):
>   m68k: Use vDSO for thread pointer access
>   m68k: Enable vDSO clock_gettime64
> 
>  sysdeps/unix/sysv/linux/m68k/m68k-helpers.c | 77 ++++++++++++++++++++-
>  sysdeps/unix/sysv/linux/m68k/sysdep.h       |  7 ++
>  2 files changed, 83 insertions(+), 1 deletion(-)
> 
>