Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754738Ab3DLLWr (ORCPT ); Fri, 12 Apr 2013 07:22:47 -0400 Received: from smtp.citrix.com ([66.165.176.89]:53476 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753061Ab3DLLWp (ORCPT ); Fri, 12 Apr 2013 07:22:45 -0400 X-IronPort-AV: E=Sophos;i="4.87,462,1363132800"; d="scan'208";a="19052622" Message-ID: <5167EE79.7060601@eu.citrix.com> Date: Fri, 12 Apr 2013 12:22:33 +0100 From: George Dunlap User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130308 Thunderbird/17.0.4 MIME-Version: 1.0 To: "linux-kernel@vger.kernel.org" CC: Konrad Wilk , Thomas Gleixner , "H. Peter Anvin" , Ingo Molnar , "x86@kernel.org" , Ian Campbell , David Vrabel , Andrew Cooper Subject: Re: [PATCH v3] perf: Check all MSRs before passing hw check References: <1365000388-32448-1-git-send-email-george.dunlap@eu.citrix.com> In-Reply-To: <1365000388-32448-1-git-send-email-george.dunlap@eu.citrix.com> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4911 Lines: 131 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 > CC: Konrad Wilk > CC: Thomas Gleixner > CC: "H. Peter Anvin" > CC: Ingo Molnar > CC: x86@kernel.org > CC: Ian Campbell > CC: David Vrabel > CC: Andrew Cooper > --- > 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; > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/