linux-kernel - Re: [PATCH] KVM: x86: never specify a sample period for virtualized in_tx

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Message-ID: <20170131073410.GC11851@gmail.com>
Date:   Tue, 31 Jan 2017 08:34:10 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Robert O'Callahan <robert@...llahan.org>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: never specify a sample period for virtualized
 in_tx_cp counters


* Robert O'Callahan <robert@...llahan.org> wrote:

> pmc_reprogram_counter() always sets a sample period based on the value of
> pmc->counter. However, hsw_hw_config() rejects sample periods less than
> 2^31 - 1. So for example, a KVM guest doing
>   perf stat -e r2005101c4 sleep 0
> will count some conditional branch events, deschedule the task, reschedule
> the task, try to restore the guest PMU state for the task, in the host
> reach pmc_reprogram_counter() with nonzero pmc->count, trigger EOPNOTSUPP
> in hsw_hw_config(), print "kvm_pmu: event creation failed" in
> pmc_reprogram_counter(), and silently (from the guest's point of view) stop
> counting events.
> 
> We fix event counting by forcing attr.sample_period to always be zero for
> in_tx_cp counters. Sampling doesn't work, but it already didn't work and
> can't be fixed without major changes to the approach in hsw_hw_config().
> 
> Signed-off-by: Robert O'Callahan <robert@...llahan.org>
> ---
>  arch/x86/kvm/pmu.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 06ce377..af993d7 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -113,12 +113,18 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>  		.config = config,
>  	};
>  
> +	attr.sample_period = (-pmc->counter) & pmc_bitmask(pmc);
> +
>  	if (in_tx)
>  		attr.config |= HSW_IN_TX;
> -	if (in_tx_cp)
> +	if (in_tx_cp) {
> +		/* HSW_IN_TX_CHECKPOINTED is not supported with nonzero
> +		 * period. Just clear the sample period so at least
> +		 * allocating the counter doesn't fail.
> +		 */
> +		attr.sample_period = 0;
>  		attr.config |= HSW_IN_TX_CHECKPOINTED;
> -
> -	attr.sample_period = (-pmc->counter) & pmc_bitmask(pmc);
> +	}

please use the customary (multi-line) comment style:

  /*
   * Comment .....
   * ...... goes here.
   */

specified in Documentation/CodingStyle.

Thanks,

        Ingo