linux-kernel - Re: [PATCH v2] x86/bugs: Explicitly clear speculative MSR bits

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Date:   Mon, 28 Nov 2022 23:40:19 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Breno Leitao <leitao@...ian.org>, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, hpa@...or.com,
        jpoimboe@...nel.org, peterz@...radead.org, x86@...nel.org,
        cascardo@...onical.com, leit@...a.com, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/bugs: Explicitly clear speculative MSR bits

On Mon, Nov 28, 2022 at 02:03:58PM -0800, Pawan Gupta wrote:
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 3e3230cccaa7..cfc2ed2661fc 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -66,7 +66,7 @@ static DEFINE_MUTEX(spec_ctrl_mutex);
>   */
>  void write_spec_ctrl_current(u64 val, bool force)
>  {
> -	if (this_cpu_read(x86_spec_ctrl_current) == val)
> +	if (!force && this_cpu_read(x86_spec_ctrl_current) == val)
>  		return;
>  	this_cpu_write(x86_spec_ctrl_current, val);

Still looks hacky to me.

I think it would be a lot cleaner if MSR_IA32_SPEC_CTRL gets cleaned of
the speculation bits in init_speculation_control() which gets run on
*every* CPU.

So by the time check_bugs() gets to setup stuff, the MSR will be ready
to go regardless.

I.e., something like this (not supposed to work - just to show what I
mean):

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 73cc546e024d..367732c92942 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -993,9 +993,19 @@ static void init_speculation_control(struct cpuinfo_x86 *c)
 	 * Intel CPUs, for finer-grained selection of what's available.
 	 */
 	if (cpu_has(c, X86_FEATURE_SPEC_CTRL)) {
+		u64 msr;
+
 		set_cpu_cap(c, X86_FEATURE_IBRS);
 		set_cpu_cap(c, X86_FEATURE_IBPB);
 		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
+
+		/*
+		 * Clear speculation control settings from a previous kernel
+		 * run, i.e., kexec.
+		 */
+		rdmsrl(MSR_IA32_SPEC_CTRL, msr);
+		if (msr & SPEC_CTRL_MASK)
+			wrmsr (MSR_IA32_SPEC_CTRL, msr & ~SPEC_CTRL_MASK);
 	}
 
 	if (cpu_has(c, X86_FEATURE_INTEL_STIBP))



-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette