linux-kernel - Re: [PATCH RFC V3 7/9] x86/entry: Preserve PKRS MSR across exceptions

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Date:   Tue, 13 Oct 2020 11:52:32 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     ira.weiny@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC V3 7/9] x86/entry: Preserve PKRS MSR across exceptions

On 10/9/20 12:42 PM, ira.weiny@...el.com wrote:
> @@ -341,6 +341,9 @@ noinstr void irqentry_enter(struct pt_regs *regs, irqentry_state_t *state)
>  	/* Use the combo lockdep/tracing function */
>  	trace_hardirqs_off();
>  	instrumentation_end();
> +
> +done:
> +	irq_save_pkrs(state);
>  }

One nit: This saves *and* sets PKRS.  It's not obvious from the call
here that PKRS is altered at this site.  Seems like there could be a
better name.

Even if we did:

	irq_save_set_pkrs(state, INIT_VAL);

It would probably compile down to the same thing, but be *really*
obvious what's going on.

>  void irqentry_exit_cond_resched(void)
> @@ -362,7 +365,12 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t *state)
>  	/* Check whether this returns to user mode */
>  	if (user_mode(regs)) {
>  		irqentry_exit_to_user_mode(regs);
> -	} else if (!regs_irqs_disabled(regs)) {
> +		return;
> +	}
> +
> +	irq_restore_pkrs(state);
> +
> +	if (!regs_irqs_disabled(regs)) {
>  		/*
>  		 * If RCU was not watching on entry this needs to be done
>  		 * carefully and needs the same ordering of lockdep/tracing
>