linux-kernel - Re: [PATCH v5 2/9] x86/split_lock: Avoid runtime reads of the TEST

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Date:   Mon, 23 Mar 2020 18:06:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xiaoyao Li <xiaoyao.li@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, hpa@...or.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v5 2/9] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

Xiaoyao Li <xiaoyao.li@...el.com> writes:
> +/*
> + * Soft copy of MSR_TEST_CTRL initialized when we first read the
> + * MSR. Used at runtime to avoid using rdmsr again just to collect
> + * the reserved bits in the MSR. We assume reserved bits are the
> + * same on all CPUs.
> + */
> +static u64 test_ctrl_val;
> +
>  /*
>   * Locking is not required at the moment because only bit 29 of this
>   * MSR is implemented and locking would not prevent that the operation
> @@ -1027,16 +1035,14 @@ static void __init split_lock_setup(void)
>   */
>  static void __sld_msr_set(bool on)
>  {
> -	u64 test_ctrl_val;
> -
> -	rdmsrl(MSR_TEST_CTRL, test_ctrl_val);
> +	u64 val = test_ctrl_val;
>  
>  	if (on)
> -		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +		val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>  	else
> -		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +		val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>  
> -	wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
> +	wrmsrl(MSR_TEST_CTRL, val);
>  }
>  
>  /*
> @@ -1048,11 +1054,13 @@ static void __sld_msr_set(bool on)
>   */
>  static void split_lock_init(struct cpuinfo_x86 *c)
>  {
> -	u64 test_ctrl_val;
> +	u64 val;
>  
> -	if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
> +	if (rdmsrl_safe(MSR_TEST_CTRL, &val))
>  		goto msr_broken;
>  
> +	test_ctrl_val = val;
> +
>  	switch (sld_state) {
>  	case sld_off:
>  		if (wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val & ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT))

That's just broken. Simply because

       case sld_warn:
       case sld_fatal:

set the split lock detect bit, but the cache variable has it cleared
unless it was set at boot time already.

Thanks,

        tglx