linux-kernel - Re: [PATCH RFC V2 04/17] x86/pks: Preserve the PKRS MSR on context switch

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Date:   Fri, 17 Jul 2020 10:31:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     ira.weiny@...el.com
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 V2 04/17] x86/pks: Preserve the PKRS MSR on context
 switch

On Fri, Jul 17, 2020 at 12:20:43AM -0700, ira.weiny@...el.com wrote:

> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index f362ce0d5ac0..d69250a7c1bf 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -42,6 +42,7 @@
>  #include <asm/spec-ctrl.h>
>  #include <asm/io_bitmap.h>
>  #include <asm/proto.h>
> +#include <asm/pkeys_internal.h>
>  
>  #include "process.h"
>  
> @@ -184,6 +185,36 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
>  	return ret;
>  }
>  
> +/*
> + * NOTE: We wrap pks_init_task() and pks_sched_in() with
> + * CONFIG_ARCH_HAS_SUPERVISOR_PKEYS because using IS_ENABLED() fails
> + * due to the lack of task_struct->saved_pkrs in this configuration.
> + * Furthermore, we place them here because of the complexity introduced by
> + * header conflicts introduced to get the task_struct definition in the pkeys
> + * headers.
> + */

I don't see anything much useful in that comment.

> +#ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
> +DECLARE_PER_CPU(u32, pkrs_cache);
> +static inline void pks_init_task(struct task_struct *tsk)
> +{
> +	/* New tasks get the most restrictive PKRS value */
> +	tsk->thread.saved_pkrs = INIT_PKRS_VALUE;
> +}
> +static inline void pks_sched_in(void)
> +{
> +	u64 current_pkrs = current->thread.saved_pkrs;
> +
> +	/* Only update the MSR when current's pkrs is different from the MSR. */
> +	if (this_cpu_read(pkrs_cache) == current_pkrs)
> +		return;
> +
> +	write_pkrs(current_pkrs);

Should we write that like:

	/*
	 * PKRS is only temporarily changed during specific code paths.
	 * Only a preemption during these windows away from the default
	 * value would require updating the MSR.
	 */
	if (unlikely(this_cpu_read(pkrs_cache) != current_pkrs))
		write_pkrs(current_pkrs);

?

> +}
> +#else
> +static inline void pks_init_task(struct task_struct *tsk) { }
> +static inline void pks_sched_in(void) { }
> +#endif