Re: [Xen-devel] [PATCH] perf: Check all MSRs before passing hw check

[Ingo Molnar <mingo@xxxxxxxxxx>]

* George Dunlap <george.dunlap@xxxxxxxxxxxxx> 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.
> 
> This changset causes check_hw_exists() to go through all of the msr
> checks, failing and returning false if any of them fail.
> 
> This problem affects kernels as far back as 3.2, and should thus be
> considered for backport.
> 
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> CC: Konrad Wilk <konrad.wilk@xxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: x86@xxxxxxxxxx
> ---
>  arch/x86/kernel/cpu/perf_event.c |   20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)

What is missing is a description of what specific platform this gets 
triggered on and exactly why. Is some hw feature emulation missing that 
causes the check to fail?

Thanks,

        Ingo

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