linux-kernel - Re: [PATCH V7 08/18] x86/entry: Preserve PKRS MSR across exceptions

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Date:   Mon, 6 Dec 2021 20:45:22 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-mm@...ck.org
Subject: Re: [PATCH V7 08/18] x86/entry: Preserve PKRS MSR across exceptions

On Mon, Dec 06, 2021 at 05:54:23PM -0800, 'Ira Weiny' wrote:

[snip]

> > 
> > Though, if you look at the xen_pv_evtchn_do_upcall() part where you
> > added this extra invocation you might figure out that adding
> > pkrs_restore_irq() to irqentry_exit_cond_resched() and explicitely to
> > the 'else' path in irqentry_exit() makes it magically consistent for
> > both use cases.
> > 
> 
> Thank you, yes good catch.  However, I think I need at least 1 more call in the
> !regs_irqs_disabled() && state.exit_rcu case right?
> 
> 11:29:48 > git di
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 717091910ebc..667676ebc129 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -363,8 +363,6 @@ __visible noinstr void xen_pv_evtchn_do_upcall(struct pt_regs *regs)
>  
>         inhcall = get_and_clear_inhcall();
>         if (inhcall && !WARN_ON_ONCE(state.exit_rcu)) {
> -               /* Normally called by irqentry_exit, restore pkrs here */
> -               pkrs_restore_irq(regs);
>                 irqentry_exit_cond_resched();
>                 instrumentation_end();
>                 restore_inhcall(inhcall);
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 1c0a70a17e93..60ae3d4c9cc0 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -385,6 +385,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
>  
>  void irqentry_exit_cond_resched(void)

Opps...  Of course regs will need to be passed in here now...

Ira

>  {
> +       pkrs_restore_irq(regs);
>         if (!preempt_count()) {
>                 /* Sanity check RCU and thread stack */
>                 rcu_irq_exit_check_preempt();
> @@ -408,8 +409,6 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>                 return;
>         }
>  
> -       pkrs_restore_irq(regs);
> -
>         if (!regs_irqs_disabled(regs)) {
>                 /*
>                  * If RCU was not watching on entry this needs to be done
> @@ -421,6 +420,7 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>                         /* Tell the tracer that IRET will enable interrupts */
>                         trace_hardirqs_on_prepare();
>                         lockdep_hardirqs_on_prepare(CALLER_ADDR0);
> +                       pkrs_restore_irq(regs);
>                         instrumentation_end();
>                         rcu_irq_exit();
>                         lockdep_hardirqs_on(CALLER_ADDR0);
> @@ -439,6 +439,10 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>                 trace_hardirqs_on();
>                 instrumentation_end();
>         } else {
> +               instrumentation_begin();
> +               pkrs_restore_irq(regs);
> +               instrumentation_end();
> +
>                 /*
>                  * IRQ flags state is correct already. Just tell RCU if it
>                  * was not watching on entry.
> @@ -470,8 +474,8 @@ irqentry_state_t noinstr irqentry_nmi_enter(struct pt_regs *regs)
>  
>  void noinstr irqentry_nmi_exit(struct pt_regs *regs, irqentry_state_t irq_state)
>  {
> -       pkrs_restore_irq(regs);
>         instrumentation_begin();
> +       pkrs_restore_irq(regs);
>         ftrace_nmi_exit();
>         if (irq_state.lockdep) {
>                 trace_hardirqs_on_prepare();
> 
> 
> Thank you again for the review,
> Ira
> 
> 
> [0] https://lore.kernel.org/lkml/20210804043231.2655537-5-ira.weiny@intel.com/
> [1] https://lore.kernel.org/lkml/20201217131924.GW3040@hirez.programming.kicks-ass.net/
> [2] https://lore.kernel.org/lkml/20201216013202.GY1563847@iweiny-DESK2.sc.intel.com/
> [3] https://lore.kernel.org/lkml/87y2hwqwng.fsf@nanos.tec.linutronix.de/
> 
> > Thanks,
> > 
> >         tglx