Re: [RFC 0/5] accel/kvm: Support KVM PMU filter

[Shaoqin Huang <shahuang@redhat.com>]

Hi Zhao,

On 7/10/24 12:51, Zhao Liu wrote:
> Hi QEMU maintainers, arm and PMU folks,
> 
> I picked up Shaoqing's previous work [1] on the KVM PMU filter for arm,
> and now is trying to support this feature for x86 with a JSON-compatible
> API.
> 
> While arm and x86 use different KVM ioctls to configure the PMU filter,
> considering they all have similar inputs (PMU event + action), it is
> still possible to abstract a generic, cross-architecture kvm-pmu-filter
> object and provide users with a sufficiently generic or near-consistent
> QAPI interface.
> 
> That's what I did in this series, a new kvm-pmu-filter object, with the
> API like:
> 
> -object '{"qom-type":"kvm-pmu-filter","id":"f0","events":[{"action":"allow","format":"raw","code":"0xc4"}]}'
> 
> For i386, this object is inserted into kvm accelerator and is extended
> to support fixed-counter and more formats ("x86-default" and
> "x86-masked-entry"):
> 
> -accel kvm,pmu-filter=f0 \
> -object pmu='{"qom-type":"kvm-pmu-filter","id":"f0","x86-fixed-counter":{"action":"allow","bitmap":"0x0"},"events":[{"action":"allow","format":"x86-masked-entry","select":"0xc4","mask":"0xff","match":"0","exclude":true},{"action":"allow","format":"x86-masked-entry","select":"0xc5","mask":"0xff","match":"0","exclude":true}]}'

What if I want to create the PMU Filter on ARM to deny the event range 
[0x5,0x10], and allow deny event 0x13, how should I write the json?

Thanks,
Shaoqin

> 
> This object can still be added as the property to the arch CPU if it is
> desired as a per CPU feature (as Shaoqin did for arm before).
> 
> Welcome your feedback and comments!
> 
> 
> Introduction
> ============
> 
> 
> Formats supported in kvm-pmu-filter
> -----------------------------------
> 
> This series supports 3 formats:
> 
> * raw format (general format).
> 
>    This format indicates the code that has been encoded to be able to
>    index the PMU events, and which can be delivered directly to the KVM
>    ioctl. For arm, this means the event code, and for i386, this means
>    the raw event with the layout like:
> 
>        select high bit | umask | select low bits
> 
> * x86-default format (i386 specific)
> 
>    x86 commonly uses select&umask to identify PMU events, and this format
>    is used to support the select&umask. Then QEMU will encode select and
>    umask into a raw format code.
> 
> * x86-masked-entry (i386 specific)
> 
>    This is a special format that x86's KVM_SET_PMU_EVENT_FILTER supports.
> 
> 
> Hexadecimal value string
> ------------------------
> 
> In practice, the values associated with PMU events (code for arm, select&
> umask for x86) are often expressed in hexadecimal. Further, from linux
> perf related information (tools/perf/pmu-events/arch/*/*/*.json), x86/
> arm64/riscv/nds32/powerpc all prefer the hexadecimal numbers and only
> s390 uses decimal value.
> 
> Therefore, it is necessary to support hexadecimal in order to honor PMU
> conventions.
> 
> However, unfortunately, standard JSON (RFC 8259) does not support
> hexadecimal numbers. So I can only consider using the numeric string in
> the QAPI and then parsing it to a number.
> 
> To achieve this, I defined two versions of PMU-related structures in
> kvm.json:
>   * a native version that accepts numeric values, which is used for
>     QEMU's internal code processing,
> 
>   * and a variant version that accepts numeric string, which is used to
>     receive user input.
> 
> kvm-pmu-filter object will take care of converting the string version
> of the event/counter information into the numeric version.
> 
> The related implementation can be found in patch 1.
> 
> 
> CPU property v.s. KVM property
> ------------------------------
> 
> In Shaoqin's previous implementation [1], KVM PMU filter is made as a
> arm CPU property. This is because arm uses a per CPU ioctl
> (KVM_SET_DEVICE_ATTR) to configure KVM PMU filter.
> 
> However, for x86, the dependent ioctl (KVM_SET_PMU_EVENT_FILTER) is per
> VM. In the meantime, considering that for hybrid architecture, maybe in
> the future there will be a new per vCPU ioctl, or there will be
> practices to support filter fixed counter by configuring CPUIDs.
> 
> Based on the above thoughts, for x86, it is not appropriate to make the
> current per-VM ioctl-based PMU filter a CPU property. Instead, I make it
> a kvm property and configure it via "-accel kvm,pmu-filter=obj_id".
> 
> So in summary, it is feasible to use the KVM PMU filter as either a CPU
> or a KVM property, depending on whether it is used as a CPU feature or a
> VM feature.
> 
> The kvm-pmu-filter object, as an abstraction, is general enough to
> support filter configurations for different scopes (per-CPU or per-VM).
> 
> 
> [1]: https://lore.kernel.org/qemu-devel/20240409024940.180107-1-shahuang@redhat.com/
> 
> Thanks and Best Regards,
> Zhao
> ---
> Zhao Liu (5):
>    qapi/qom: Introduce kvm-pmu-filter object
>    i386/kvm: Support initial KVM PMU filter
>    i386/kvm: Support event with select&umask format in KVM PMU filter
>    i386/kvm: Support event with masked entry format in KVM PMU filter
>    i386/kvm: Support fixed counter in KVM PMU filter
> 
>   MAINTAINERS                |   1 +
>   accel/kvm/kvm-pmu.c        | 367 +++++++++++++++++++++++++++++++++++++
>   accel/kvm/meson.build      |   1 +
>   include/sysemu/kvm-pmu.h   |  43 +++++
>   include/sysemu/kvm_int.h   |   2 +
>   qapi/kvm.json              | 255 ++++++++++++++++++++++++++
>   qapi/meson.build           |   1 +
>   qapi/qapi-schema.json      |   1 +
>   qapi/qom.json              |   3 +
>   target/i386/kvm/kvm.c      | 211 +++++++++++++++++++++
>   target/i386/kvm/kvm_i386.h |   1 +
>   11 files changed, 886 insertions(+)
>   create mode 100644 accel/kvm/kvm-pmu.c
>   create mode 100644 include/sysemu/kvm-pmu.h
>   create mode 100644 qapi/kvm.json
> 

-- 
Shaoqin