Re: [RFC PATCH 1/5] entry: Pass pt_regs to irqentry_exit_cond_resched()

[Borislav Petkov]

On Fri, Aug 05, 2022 at 10:30:05AM -0700, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> Auxiliary pt_regs space needs to be manipulated by the generic
> entry/exit code.
> 
> Ideally irqentry_exit() would take care of handling any auxiliary
> pt_regs on exit.  Unfortunately, irqentry_exit() is not the only exit
> from exception path.  The call to irqentry_exit_cond_resched() from
> xen_pv_evtchn_do_upcall() bypasses irqentry_exit().
> 
> Make irqentry_exit_cond_resched() symmetrical with irqentry_enter() by
> passing pt_regs to it.  This makes irqentry_exit_cond_resched() capable
> of handling auxiliary pt_regs in future patches.
> 
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> ---
> Forward ported from PKS series:
> 	https://lore.kernel.org/lkml/20220419170649.1022246-19-ira.weiny@xxxxxxxxx/
> ---
>  arch/arm64/include/asm/preempt.h |  2 +-
>  arch/arm64/kernel/entry-common.c |  4 ++--
>  arch/x86/entry/common.c          |  2 +-
>  include/linux/entry-common.h     | 17 ++++++++------
>  kernel/entry/common.c            | 13 +++++++----
>  kernel/sched/core.c              | 40 ++++++++++++++++----------------
>  6 files changed, 43 insertions(+), 35 deletions(-)

Why all this churn?

Why can't you add a parameter to irqentry_exit():

  noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state, bool cond_resched);

and then have all callers except xen_pv_evtchn_do_upcall() pass in false
and this way have all exit paths end up in irqentry_exit()?

And, ofc, move the true case which is the body of
raw_irqentry_exit_cond_resched() to irqentry_exit() and then get rid of
former.

xen_pv_evtchn_do_upcall() will, ofc, do:

        if (inhcall && !WARN_ON_ONCE(state.exit_rcu)) {
                irqentry_exit(regs, state, true);
                instrumentation_end();
                restore_inhcall(inhcall);
        } else {
                instrumentation_end();
                irqentry_exit(regs, state, false);

Hmmm?

-- 
Regards/Gruss,
    Boris.

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