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Date: Wed, 9 Jul 2014 19:32:09 +0000 From: "Liang, Kan" <kan.liang@...el.com> To: Peter Zijlstra <peterz@...radead.org> CC: "andi@...stfloor.org" <andi@...stfloor.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org> Subject: RE: [PATCH V4 1/2] perf ignore LBR and extra_regs. > > > > > > On Wed, Jul 09, 2014 at 02:32:28PM +0000, Liang, Kan wrote: > > > > > > > > > > > > > On Tue, Jul 08, 2014 at 09:49:40AM -0700, kan.liang@...el.com wrote: > > > > > > +/* > > > > > > + * Under certain circumstances, access certain MSR may cause #GP. > > > > > > + * The function tests if the input MSR can be safely accessed. > > > > > > + */ > > > > > > +static inline bool check_msr(unsigned long msr) { > > > > > > + u64 value; > > > > > > + > > > > > > + if (rdmsrl_safe(msr, &value) < 0) > > > > > > + return false; > > > > > > + if (wrmsrl_safe(msr, value) < 0) > > > > > > + return false; > > > > > > + return true; > > > > > > +} > > > > > > > > > > What does this thing return after patch 2? does the write still > > > > > fault or will KVM silently take writes too? > > > > > > > > If applying patch 2, the function will return true. The KVM just > > > > simply ignore > > > the reads/writes. > > > > > > OK, then that's broken too. We want a function to return false for > > > any malfunctioning MSR, ignoring writes and returning 0s is not > > > proper functioning. > > > > The patch 2 is to handle the case that the administrator can only > > patch the host. Don't need to force user to upgrade their guest to fix > > the crash. And ignore the annoying "unhandled...." KVM messages > > Sure; but what I meant was, check_msr() is broken when ran on such a > kernel. You need to fix check_msr() to return failure on these 'ignored' > MSRs, after all they don't function as expected, they're effectively broken. The function is designed to check if the MSRs can be safely accessed (no #GP). It cannot guarantee the correctness of the MSRs. If KVM applied patch 2 and guest applied patch 1, from the guest's perspective, the MSRs can be accessed (no #GP triggered). So return true is expected. It should not be a broken. The only unexpected thing for guest is that the counting/sampling result for LBR/extra reg is always 0. But the patch is a short term fix to stop things from crashing. I think it should be acceptable. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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