[PATCH v2] arm64: Implement prctl(PR_{G,S}ET_TSC)

[Peter Collingbourne]

On Fri, Aug 23, 2024 at 5:01 AM Will Deacon <will at kernel.org> wrote:
>
> On Fri, May 17, 2024 at 02:25:51PM -0700, Peter Collingbourne wrote:
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 4ae31b7af6c31..1a2ae7830c179 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -43,6 +43,7 @@
> >  #include <linux/stacktrace.h>
> >
> >  #include <asm/alternative.h>
> > +#include <asm/arch_timer.h>
> >  #include <asm/compat.h>
> >  #include <asm/cpufeature.h>
> >  #include <asm/cacheflush.h>
> > @@ -472,27 +473,49 @@ static void entry_task_switch(struct task_struct *next)
> >  }
> >
> >  /*
> > - * ARM erratum 1418040 handling, affecting the 32bit view of CNTVCT.
> > - * Ensure access is disabled when switching to a 32bit task, ensure
> > - * access is enabled when switching to a 64bit task.
> > + * Handle sysreg updates for ARM erratum 1418040 which affects the 32bit view of
> > + * CNTVCT, various other errata which require trapping all CNTVCT{,_EL0}
> > + * accesses and prctl(PR_SET_TSC). Ensure access is disabled iff a workaround is
> > + * required or PR_TSC_SIGSEGV is set.
> >   */
> > -static void erratum_1418040_thread_switch(struct task_struct *next)
> > +static void update_cntkctl_el1(struct task_struct *next)
> >  {
> > -     if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) ||
> > -         !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
> > -             return;
> > +     struct thread_info *ti = task_thread_info(next);
> >
> > -     if (is_compat_thread(task_thread_info(next)))
> > +     if (test_ti_thread_flag(ti, TIF_TSC_SIGSEGV) ||
> > +         has_erratum_handler(read_cntvct_el0) ||
> > +         (IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) &&
> > +          this_cpu_has_cap(ARM64_WORKAROUND_1418040) &&
> > +          is_compat_thread(ti)))
> >               sysreg_clear_set(cntkctl_el1, ARCH_TIMER_USR_VCT_ACCESS_EN, 0);
> >       else
> >               sysreg_clear_set(cntkctl_el1, 0, ARCH_TIMER_USR_VCT_ACCESS_EN);
> >  }
> >
> > -static void erratum_1418040_new_exec(void)
> > +static void cntkctl_thread_switch(struct task_struct *prev,
> > +                               struct task_struct *next)
> > +{
> > +     if ((read_ti_thread_flags(task_thread_info(prev)) &
> > +          (_TIF_32BIT | _TIF_TSC_SIGSEGV)) !=
> > +         (read_ti_thread_flags(task_thread_info(next)) &
> > +          (_TIF_32BIT | _TIF_TSC_SIGSEGV)))
> > +             update_cntkctl_el1(next);
> > +}
> > +
> > +static int do_set_tsc_mode(unsigned int val)
> >  {
> > +     if (val == PR_TSC_SIGSEGV)
> > +             set_thread_flag(TIF_TSC_SIGSEGV);
> > +     else if (val == PR_TSC_ENABLE)
> > +             clear_thread_flag(TIF_TSC_SIGSEGV);
> > +     else
> > +             return -EINVAL;
>
> Aren't we in trouble if we get preempted at this point? The flag is
> out-of-sync with the register, so I worry that we could fail to set the
> controls correctly for the next task.

I think you're right; fixed in v3.

Peter