Messages in this thread | | | Date | Thu, 28 Sep 2023 17:24:37 +0800 | Subject | Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics event | From | "Mi, Dapeng" <> |
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On 9/28/2023 1:27 AM, Sean Christopherson wrote: > +Jim, David, and Mingwei > > On Wed, Sep 27, 2023, Peter Zijlstra wrote: >> On Wed, Sep 27, 2023 at 11:31:18AM +0800, Dapeng Mi wrote: >>> When guest wants to use PERF_METRICS MSR, a virtual metrics event needs >>> to be created in the perf subsystem so that the guest can have exclusive >>> ownership of the PERF_METRICS MSR. >> Urgh, can someone please remind me how all that is supposed to work >> again? The guest is just a task that wants the event. If the >> host creates a CPU event, then that gets scheduled with higher priority >> and the task looses out, no joy.
It looks I used the inaccurate words in the comments. Yes, it's not *exclusive* from host's point view. Currently the perf events created by KVM are task-pinned events, they are indeed possible to be preempted by CPU-pinned host events which have higher priority. This is a long term issue which vPMU encountered. We ever have some internal discussion about this issue, but it seems we don't have a good way to solve this issue thoroughly in current vPMU framework.
But if there is no such CPU-pinned events which have the highest priority on host, KVM perf events can share the HW resource with other host events with the way of time-multiplexing.
>> So you cannot guarantee the guest gets anything. >> >> That is, I remember we've had this exact problem before, but I keep >> forgetting how this all is supposed to work. I don't use this virt stuff >> (and every time I try qemu arguments defeat me and I give up in >> disgust). > I don't think it does work, at least not without a very, very carefully crafted > setup and a host userspace that knows it must not use certain aspects of perf. > E.g. for PEBS, if the guest virtual counters don't map 1:1 to the "real" counters > in hardware, KVM+perf simply disables the counter. > > And for top-down slots, getting anything remotely accurate requires pinning vCPUs > 1:1 with pCPUs and enumerating an accurate toplogy to the guest: > > The count is distributed among unhalted logical processors (hyper-threads) who > share the same physical core, in processors that support Intel Hyper-Threading > Technology. > > Jumping the gun a bit (we're in the *super* early stages of scraping together a > rough PoC), but I think we should effectively put KVM's current vPMU support into > maintenance-only mode, i.e. stop adding new features unless they are *very* simple > to enable, and instead pursue an implementation that (a) lets userspace (and/or > the kernel builder) completely disable host perf (or possibly just host perf usage > of the hardware PMU) and (b) let KVM passthrough the entire hardware PMU when it > has been turned off in the host. > > I.e. keep KVM's existing best-offset vPMU support, e.g. for setups where the > platform owner is also the VM ueer (running a Windows VM on a Linux box, hosting > a Linux VM in ChromeOS, etc...). But for anything advanced and for hard guarantees, > e.g. cloud providers that want to expose fully featured vPMU to customers, force > the platform owner to choose between using perf (or again, perf with hardware PMU) > in the host, and exposing the hardware PMU to the guest. > > Hardware vendors are pushing us in the direction whether we like it or not, e.g. > SNP and TDX want to disallow profiling the guest from the host, ARM has an > upcoming PMU model where (IIUC) it can't be virtualized without a passthrough > approach, Intel's hybrid CPUs are a complete trainwreck unless vCPUs are pinned, > and virtualizing things like top-down slots, PEBS, and LBRs in the shared model > requires an absurd amount of complexity throughout the kernel and userspace. > > Note, a similar idea was floated and rejected in the past[*], but that failed > proposal tried to retain host perf+PMU functionality by making the behavior dynamic, > which I agree would create an awful ABI for the host. If we make the "knob" a > Kconfig or kernel param, i.e. require the platform owner to opt-out of using perf > no later than at boot time, then I think we can provide a sane ABI, keep the > implementation simple, all without breaking existing users that utilize perf in > the host to profile guests. > > [*] https://lore.kernel.org/all/CALMp9eRBOmwz=mspp0m5Q093K3rMUeAsF3vEL39MGV5Br9wEQQ@mail.gmail.com
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