Re: [patch 21/24] x86/speculation: Prepare arch_smt_update() for PRCTL mode

From: Ingo Molnar <mingo@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jiri Kosina <jkosina@suse.cz>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Andi Kleen <ak@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Casey Schaufler <casey.schaufler@intel.com>,
	Asit Mallick <asit.k.mallick@intel.com>,
	Arjan van de Ven <arjan@linux.intel.com>,
	Jon Masters <jcm@redhat.com>, Waiman Long <longman9394@gmail.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	Dave Stewart <david.c.stewart@intel.com>,
	Kees Cook <keescook@chromium.org>
Subject: Re: [patch 21/24] x86/speculation: Prepare arch_smt_update() for PRCTL mode
Date: Thu, 22 Nov 2018 08:34:47 +0100	[thread overview]
Message-ID: <20181122073447.GD41788@gmail.com> (raw)
In-Reply-To: <20181121201724.320605317@linutronix.de>

* Thomas Gleixner <tglx@linutronix.de> wrote:

> The upcoming fine grained per task STIBP control needs to be updated on CPU
> hotplug as well.
> 
> Split out the code which controls the strict mode so the prctl control code
> can be added later.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/x86/kernel/cpu/bugs.c |   46 ++++++++++++++++++++++++---------------------
>  1 file changed, 25 insertions(+), 21 deletions(-)
> 
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -531,40 +531,44 @@ static void __init spectre_v2_select_mit
>  	arch_smt_update();
>  }
>  
> -static bool stibp_needed(void)
> +static void update_stibp_msr(void *info)
>  {
> -	/* Enhanced IBRS makes using STIBP unnecessary. */
> -	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
> -		return false;
> -
> -	/* Check for strict app2app mitigation mode */
> -	return spectre_v2_app2app == SPECTRE_V2_APP2APP_STRICT;
> +	wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
>  }

Does Sparse or other tooling warn about unused function parameters? If 
yes then it might make sense to mark it __used?

>  
> -static void update_stibp_msr(void *info)
> +/* Update x86_spec_ctrl_base in case SMT state changed. */
> +static void update_stibp_strict(void)
>  {
> -	wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> +	u64 mask = x86_spec_ctrl_base & ~SPEC_CTRL_STIBP;
> +
> +	if (sched_smt_active())
> +		mask |= SPEC_CTRL_STIBP;
> +
> +	if (mask == x86_spec_ctrl_base)
> +		return;
> +
> +	pr_info("Spectre v2 cross-process SMT mitigation: %s STIBP\n",
> +		mask & SPEC_CTRL_STIBP ? "Enabling" : "Disabling");
> +	x86_spec_ctrl_base = mask;
> +	on_each_cpu(update_stibp_msr, NULL, 1);
>  }
>  
>  void arch_smt_update(void)
>  {
> -	u64 mask;
> -
> -	if (!stibp_needed())
> +	/* Enhanced IBRS makes using STIBP unnecessary. No update required. */
> +	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
>  		return;
>  
>  	mutex_lock(&spec_ctrl_mutex);
>  
> -	mask = x86_spec_ctrl_base & ~SPEC_CTRL_STIBP;
> -	if (sched_smt_active())
> -		mask |= SPEC_CTRL_STIBP;
> -
> -	if (mask != x86_spec_ctrl_base) {
> -		pr_info("Spectre v2 cross-process SMT mitigation: %s STIBP\n",
> -			mask & SPEC_CTRL_STIBP ? "Enabling" : "Disabling");
> -		x86_spec_ctrl_base = mask;
> -		on_each_cpu(update_stibp_msr, NULL, 1);
> +	switch (spectre_v2_app2app) {
> +	case SPECTRE_V2_APP2APP_NONE:
> +		break;
> +	case SPECTRE_V2_APP2APP_STRICT:
> +		update_stibp_strict();
> +		break;
>  	}

So I'm wondering, shouldn't firmware_restrict_branch_speculation_start()/_end()
also enable/disable STIBP? It already enabled/disables IBRS.

Thanks,

	Ingo