Re: [PATCH v3] perf: Check all MSRs before passing hw check

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

Any thoughts on this?

Thanks,
  -George

On 03/04/13 15:46, 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.
>
> v3:
>   - Save the register and value which failed, and print them once at the end.
> 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..2456bae 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -180,8 +180,9 @@ static void release_pmc_hardware(void) {}
>   
>   static bool check_hw_exists(void)
>   {
> -	u64 val, val_new = ~0;
> -	int i, reg, ret = 0;
> +	u64 val, val_fail, val_new= ~0;
> +	int i, reg, reg_fail, ret = 0;
> +	int bios_fail = 0;
>   
>   	/*
>   	 * Check to see if the BIOS enabled any of the counters, if so
> @@ -192,8 +193,11 @@ 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 = 1;
> +			val_fail = val;
> +			reg_fail = reg;
> +		}
>   	}
>   
>   	if (x86_pmu.num_counters_fixed) {
> @@ -202,8 +206,11 @@ static bool check_hw_exists(void)
>   		if (ret)
>   			goto msr_fail;
>   		for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
> -			if (val & (0x03 << i*4))
> -				goto bios_fail;
> +			if (val & (0x03 << i*4)) {
> +				bios_fail = 1;
> +				val_fail = val;
> +				reg_fail = reg;
> +			}
>   		}
>   	}
>   
> @@ -221,14 +228,13 @@ static bool check_hw_exists(void)
>   	if (ret || val != val_new)
>   		goto msr_fail;
>   
> -	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);
> +	 */ 						
> +	if (bios_fail) {
> +		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_fail, val_fail);
> +	}
>   
>   	return true;
>