linux-kernel - Re: [PATCH RFC V3 4/9] x86/pks: Preserve the PKRS MSR on context switch

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Message-ID: <20201016110636.GL2611@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Oct 2020 13:06:36 +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>,
        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 V3 4/9] x86/pks: Preserve the PKRS MSR on context
 switch

On Fri, Oct 09, 2020 at 12:42:53PM -0700, ira.weiny@...el.com wrote:

> @@ -644,6 +663,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
>  
>  	if ((tifp ^ tifn) & _TIF_SLD)
>  		switch_to_sld(tifn);
> +
> +	pks_sched_in();
>  }
>  

You seem to have lost the comment proposed here:

  https://lkml.kernel.org/r/20200717083140.GW10769@hirez.programming.kicks-ass.net

It is useful and important information that the wrmsr normally doesn't
happen.

> diff --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
> index 3cf8f775f36d..30f65dd3d0c5 100644
> --- a/arch/x86/mm/pkeys.c
> +++ b/arch/x86/mm/pkeys.c
> @@ -229,3 +229,31 @@ u32 update_pkey_val(u32 pk_reg, int pkey, unsigned int flags)
>  
>  	return pk_reg;
>  }
> +
> +DEFINE_PER_CPU(u32, pkrs_cache);
> +
> +/**
> + * It should also be noted that the underlying WRMSR(MSR_IA32_PKRS) is not
> + * serializing but still maintains ordering properties similar to WRPKRU.
> + * The current SDM section on PKRS needs updating but should be the same as
> + * that of WRPKRU.  So to quote from the WRPKRU text:
> + *
> + * 	WRPKRU will never execute transiently. Memory accesses
> + * 	affected by PKRU register will not execute (even transiently)
> + * 	until all prior executions of WRPKRU have completed execution
> + * 	and updated the PKRU register.

(whitespace damage; space followed by tabstop)

> + */
> +void write_pkrs(u32 new_pkrs)
> +{
> +	u32 *pkrs;
> +
> +	if (!static_cpu_has(X86_FEATURE_PKS))
> +		return;
> +
> +	pkrs = get_cpu_ptr(&pkrs_cache);
> +	if (*pkrs != new_pkrs) {
> +		*pkrs = new_pkrs;
> +		wrmsrl(MSR_IA32_PKRS, new_pkrs);
> +	}
> +	put_cpu_ptr(pkrs);
> +}

looks familiar that... :-)