Re: [PATCH v2] perf: Check all MSRs before passing hw check

[George Dunlap <george.dunlap@eu.citrix.com>]

Any opinions on this?  It's been 2 weeks now.

  -George

On 18/03/13 11:27, George Dunlap wrote:
> check_hw_exists has a number of checks which go to two exit paths:
> msr_fail and bios_fail.  Checks classified as msr_fail will cause
> check_hw_exists() to return false, causing the PMU not to be used;
> bios_fail checks will only cause a warning to be printed, but will
> return true.
>
> The problem is that if there are both msr failures and bios failures,
> and the routine hits a bios_fail check first, it will exit early and
> return true, not finishing the rest of the msr checks.  If those msrs
> are in fact broken, it will cause them to be used erroneously.
>
> In the case of a Xen PV VM, the guest OS has read access to all the
> MSRs, but write access is white-listed to supported features.  Writes
> to unsupported MSRs have no effect.  The PMU MSRs are not (typically)
> supported, because they are expensive to save and restore on a VM
> context switch.  One of the "msr_fail" checks is supposed to detect
> this circumstance (ether for Xen or KVM) and disable the harware PMU.
>
> However, on one of my AMD boxen, there is (apparently) a broken BIOS
> which triggers one of the bios_fail checks.  In particular,
> MSR_K7_EVNTSEL0 has the ARCH_PERFMON_EVENTSEL_ENABLE bit set.  The
> guest kernel detects this because it has read access to all MSRs, and
> causes it to skip the rest of the checks and try to use the
> non-existent hardware PMU.  This minimally causes a lot of useless
> instruction emulation and Xen console spam; it may cause other issues
> with the watchdog as well.
>
> This changset causes check_hw_exists() to go through all of the msr
> checks, failing and returning false if any of them fail.  This makes
> sure that a guest running under Xen without a virtual PMU will detect
> that there is no functioning PMU and not attempt to use it.
>
> This problem affects kernels as far back as 3.2, and should thus be
> considered for backport.
>
> v2:
>   - Print the warning when the event happens so the reg,val make sense
>   - But print it only for the first such instance
>   - Update changelog to include details of failing system
>
> Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
> CC: Konrad Wilk <konrad.wilk@oracle.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: x86@kernel.org
> CC: Ian Campbell <ian.campbell@citrix.com>
> CC: David Vrabel <david.vrabel@citrix.com>
> CC: Andrew Cooper <andrew.cooper3@citrix.com>



> ---
>   arch/x86/kernel/cpu/perf_event.c |   30 ++++++++++++++++++------------
>   1 file changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 6774c17..5e2b4d6 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -182,6 +182,19 @@ static bool check_hw_exists(void)
>   {
>   	u64 val, val_new = ~0;
>   	int i, reg, ret = 0;
> +	int bios_fail = 0;
> +
> +#define BIOS_FAIL(_r, _v) \
> +do {							\
> +	/* 						\
> +	 * We still allow the PMU driver to operate: 	\
> +	 */ 						\
> +	if (!bios_fail) { 				\
> +		bios_fail = 1; 				\
> +		printk(KERN_CONT "Broken BIOS detected, complain to your hardware vendor.\n"); \
> +		printk(KERN_ERR FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n", _r, _v); \
> +        } 						\
> +} while(0)
>   
>   	/*
>   	 * Check to see if the BIOS enabled any of the counters, if so
> @@ -192,8 +205,8 @@ static bool check_hw_exists(void)
>   		ret = rdmsrl_safe(reg, &val);
>   		if (ret)
>   			goto msr_fail;
> -		if (val & ARCH_PERFMON_EVENTSEL_ENABLE)
> -			goto bios_fail;
> +		if (val & ARCH_PERFMON_EVENTSEL_ENABLE)
> +			BIOS_FAIL(reg, val);
>   	}
>   
>   	if (x86_pmu.num_counters_fixed) {
> @@ -203,10 +216,12 @@ static bool check_hw_exists(void)
>   			goto msr_fail;
>   		for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
>   			if (val & (0x03 << i*4))
> -				goto bios_fail;
> +				BIOS_FAIL(reg, val);
>   		}
>   	}
>   
> +#undef BIOS_FAIL
> +
>   	/*
>   	 * Read the current value, change it and read it back to see if it
>   	 * matches, this is needed to detect certain hardware emulators
> @@ -223,15 +238,6 @@ static bool check_hw_exists(void)
>   
>   	return true;
>   
> -bios_fail:
> -	/*
> -	 * We still allow the PMU driver to operate:
> -	 */
> -	printk(KERN_CONT "Broken BIOS detected, complain to your hardware vendor.\n");
> -	printk(KERN_ERR FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n", reg, val);
> -
> -	return true;
> -
>   msr_fail:
>   	printk(KERN_CONT "Broken PMU hardware detected, using software events only.\n");
>   	printk(KERN_ERR "Failed to access perfctr msr (MSR %x is %Lx)\n", reg, val_new);