Messages in this thread | | | Date | Fri, 21 Jun 2019 17:22:09 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH v7 07/18] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on |
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On Wed, 8 May 2019, Chang S. Bae wrote: > From: Andy Lutomirski <luto@kernel.org> > > With the new FSGSBASE instructions, we can efficiently read and write > the FSBASE and GSBASE in __switch_to(). Use that capability to preserve > the full state. > > This will enable user code to do whatever it wants with the new > instructions without any kernel-induced gotchas. (There can still be > architectural gotchas: movl %gs,%eax; movl %eax,%gs may change GSBASE > if WRGSBASE was used, but users are expected to read the CPU manual > before doing things like that.) > > This is a considerable speedup. It seems to save about 100 cycles > per context switch compared to the baseline 4.6-rc1 behavior on my > Skylake laptop. > > [ chang: 5~10% performance improvements were seen by a context switch > benchmark that ran threads with different FS/GSBASE values (to the > baseline 4.16). Minor edit on the changelog. ] > > Signed-off-by: Andy Lutomirski <luto@kernel.org> > Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com> > Reviewed-by: Andi Kleen <ak@linux.intel.com> > Cc: H. Peter Anvin <hpa@zytor.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@kernel.org> > --- > arch/x86/kernel/process_64.c | 34 ++++++++++++++++++++++++++++------ > 1 file changed, 28 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c > index 17421c3..e2089c9 100644 > --- a/arch/x86/kernel/process_64.c > +++ b/arch/x86/kernel/process_64.c > @@ -239,8 +239,18 @@ static __always_inline void save_fsgs(struct task_struct *task) > { > savesegment(fs, task->thread.fsindex); > savesegment(gs, task->thread.gsindex); > - save_base_legacy(task, task->thread.fsindex, FS); > - save_base_legacy(task, task->thread.gsindex, GS); > + if (static_cpu_has(X86_FEATURE_FSGSBASE)) { > + /* > + * If FSGSBASE is enabled, we can't make any useful guesses > + * about the base, and user code expects us to save the current > + * value. Fortunately, reading the base directly is efficient. > + */ > + task->thread.fsbase = rdfsbase(); > + task->thread.gsbase = __rdgsbase_inactive();
This explodes when called from copy_thread_tls() when an interrupt hits after switching GS in __rdgsbase_inactive(). Wrapped it with local_irq_save/restore().
Oh well.
Thanks,
tglx
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