From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joerg Roedel Subject: Re: [PATCH v1 0/5] KVM in-guest performance monitoring Date: Fri, 13 May 2011 14:34:34 +0200 Message-ID: <20110513123434.GK8707@8bytes.org> References: <1305129333-7456-1-git-send-email-avi@redhat.com> <20110512093309.GD8707@8bytes.org> <4DCBACC7.8080000@siemens.com> <20110512131130.GG8707@8bytes.org> <4DCBE13A.2050204@redhat.com> <20110512142449.GI8707@8bytes.org> <4DCBF0B7.7080102@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kiszka , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Arnaldo Carvalho de Melo To: Avi Kivity Return-path: Content-Disposition: inline In-Reply-To: <4DCBF0B7.7080102@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Thu, May 12, 2011 at 05:37:43PM +0300, Avi Kivity wrote: > On 05/12/2011 05:24 PM, Joerg Roedel wrote: > One thing we could do is paravirtualize at a lower level, introduce a > hypercall for batch MSR reads and writes. So we can use the existing > PMU semantics and code, just optimize the switch. This is similar to > what Xen did with lazy cpu updates, and what kvm did for paravirt > pagetable writes. In general a good idea. For MSR writes this isn't trivial as well. MSR reads and writes can cause a #GP, this needs to be represented in this interface. Can we allow that a batched MSR write changes the operation mode of the vcpu and so on. Or we limit this interface to the PMU MSRs, then we are pretty much at a paravirt-pmu. > I've considered something similar for mmio - use hypercalls for ordinary > mmio to avoid calling into the emulator - but virtio uses pio which > isn't emulated and we don't have massive consumers of mmio (except > perhaps hpet). > > (and we can have a cpuid bit that advertises whether we recommend to use > this feature for PMU MSRs; if/when we get hardware support, we turn it > off) A cpuid-bit that indicates which pmu is prefered is certainly a good idea. Joerg