Re: [PATCH] x86/perf: Fix virtualization sanity check

[Konrad Rzeszutek Wilk]

On Tue, Oct 09, 2012 at 05:38:34PM +0200, Andre Przywara wrote:
> In check_hw_exists() we try to detect non-emulated MSR accesses
> by writing an arbitrary value into one of the PMU registers
> and check if it's value after a readout is still the same.
> This algorithm silently assumes that the register does not contain
> the magic value already, which is wrong in at least one situation.
> 
> Fix the algorithm to really do a read-modify-write cycle. This fixes
> a warning under Xen under some circumstances on AMD family 10h CPUs.
> 
> The reasons in more details actually sound like a story from
> Believe It or Not!:
> First you need an AMD family 10h/12h CPU. These do not reset the
> PERF_CTR registers on a reboot.
> Now you boot bare metal Linux, which goes successfully through this
> check, but leaves the magic value of 0xabcd in the register. You
> don't use the performance counters, but do a reboot (warm reset).
> Then you choose to boot Xen. The check will be triggered with a
> recent Linux kernel as Dom0 again, trying to write 0xabcd into the
> MSR. Xen silently drops the write (expected), but the subsequent read
> will return the value in the register, which just happens to be the
> expected magic value. Thus the test misleadingly succeeds, leaving

Is that an oversight in the hypervisor? as in should it disable
access to those MSRs? I thought it disabled to most of them already
unless you give some extra bootup parameters? (cpufreq=dom0 or something
like that).

> the kernel in the belief that the PMU is available. This will trigger
> the following message:
> 
> [    0.020294] ------------[ cut here ]------------
> [    0.020311] WARNING: at arch/x86/xen/enlighten.c:730 xen_apic_write+0x15/0x17()
> [    0.020318] Hardware name: empty
> [    0.020323] Modules linked in:
> [    0.020334] Pid: 1, comm: swapper/0 Not tainted 3.3.8 #7
> [    0.020340] Call Trace:
> [    0.020354]  [<ffffffff81050379>] warn_slowpath_common+0x80/0x98
> [    0.020369]  [<ffffffff810503a6>] warn_slowpath_null+0x15/0x17
> [    0.020378]  [<ffffffff810034df>] xen_apic_write+0x15/0x17
> [    0.020392]  [<ffffffff8101cb2b>] perf_events_lapic_init+0x2e/0x30
> [    0.020410]  [<ffffffff81ee4dd0>] init_hw_perf_events+0x250/0x407
> [    0.020419]  [<ffffffff81ee4b80>] ? check_bugs+0x2d/0x2d
> [    0.020430]  [<ffffffff81002181>] do_one_initcall+0x7a/0x131
> [    0.020444]  [<ffffffff81edbbf9>] kernel_init+0x91/0x15d
> [    0.020456]  [<ffffffff817caaa4>] kernel_thread_helper+0x4/0x10
> [    0.020471]  [<ffffffff817c347c>] ? retint_restore_args+0x5/0x6
> [    0.020481]  [<ffffffff817caaa0>] ? gs_change+0x13/0x13
> [    0.020500] ---[ end trace a7919e7f17c0a725 ]---
> 
> The new code will change every of the 16 low bits read from the
> register and tries to write and read-back that modified number
> from the MSR.
> 
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
> ---
>  arch/x86/kernel/cpu/perf_event.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 915b876..d18b2b8 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -208,12 +208,14 @@ static bool check_hw_exists(void)
>  	}
>  
>  	/*
> -	 * Now write a value and read it back to see if it matches,
> -	 * this is needed to detect certain hardware emulators (qemu/kvm)
> -	 * that don't trap on the MSR access and always return 0s.
> +	 * Read the current value, change it and read it back to see if it
> +	 * matches, this is needed to detect certain hardware emulators
> +	 * (qemu/kvm) that don't trap on the MSR access and always return 0s.
>  	 */
> -	val = 0xabcdUL;
>  	reg = x86_pmu_event_addr(0);
> +	if (rdmsrl_safe(reg, &val))
> +		goto msr_fail;
> +	val ^= 0xffffUL;
>  	ret = wrmsrl_safe(reg, val);
>  	ret |= rdmsrl_safe(reg, &val_new);
>  	if (ret || val != val_new)
> -- 
> 1.7.12.1
> 
> 
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