Re: [PATCH 1/5] qapi/qom: Introduce kvm-pmu-filter object

[Markus Armbruster <armbru@redhat.com>]

Zhao Liu <zhao1.liu@intel.com> writes:

> On Fri, Apr 11, 2025 at 06:38:35AM +0200, Markus Armbruster wrote:
>> Date: Fri, 11 Apr 2025 06:38:35 +0200
>> From: Markus Armbruster <armbru@redhat.com>
>> Subject: Re: [PATCH 1/5] qapi/qom: Introduce kvm-pmu-filter object
>> 
>> Zhao Liu <zhao1.liu@intel.com> writes:
>> 
>> > Hi Markus
>> >
>> > On Thu, Apr 10, 2025 at 04:21:01PM +0200, Markus Armbruster wrote:
>> >> Date: Thu, 10 Apr 2025 16:21:01 +0200
>> >> From: Markus Armbruster <armbru@redhat.com>
>> >> Subject: Re: [PATCH 1/5] qapi/qom: Introduce kvm-pmu-filter object
>> >> 
>> >> Zhao Liu <zhao1.liu@intel.com> writes:
>> >> 
>> >> > Introduce the kvm-pmu-filter object and support the PMU event with raw
>> >> > format.
>> >> 
>> >> Remind me, what does the kvm-pmu-filter object do, and why would we
>> >> want to use it?
>> >
>> > KVM PMU filter allows user space to set PMU event whitelist / blacklist
>> > for Guest. Both ARM and x86's KVMs accept a list of PMU events, and x86
>> > also accpets other formats & fixed counter field.
>> 
>> But what does the system *do* with these event lists?
>
> This is for security purposes, and can restrict Guest users from
> accessing certain sensitive hardware information on the Host via perf or
> PMU counter.
>
> When a PMU event is blocked by KVM, Guest users can't get the
> corresponding event count via perf/PMU counter.
>
> EMM, if ‘system’ refers to the QEMU part, then QEMU is responsible
> for checking the format and passing the list to KVM.
>
> Thanks,
> Zhao

This helped some, thanks.  To make sure I got it:

KVM can restrict the guest's access to the PMU.  This is either a
whitelist (guest can access exactly what's on this list), or a blacklist
(guest can access exactly what's not this list).

QEMU's kvm-pmu-filter object provides an interface to this KVM feature.

KVM takes "raw" list entries: an entry is a number, and the number's
meaning depends on the architecture.  The kvm-pmu-filter object can take
such entries, and passes them to straight to KVM.

On x86, we commonly use two slightly higher level formats: select &
umask, and masked.  The kvm-pmu-filter object can take entries in either
format, and maps them to "raw".

Correct?