Re: [RFC PATCH 00/38] arm_mpam: Add KVM/arm64 and resctrl glue code

Ben Horgan
2025-12-09T16:14:17Z

Hi Peter,

On 12/9/25 15:53, Peter Newman wrote:
> Hi Ben,
>
> On Tue, Dec 9, 2025 at 3:40 PM Ben Horgan <ben.horgan@arm.com> wrote:
>>
>> Hi James and all,
[...]
>>
>> One major departure from the previous snapshot branches referenced in
>> the base driver series is that the same MPAM setting are used for
>> kernel-space and user-space. That is, MPAM1_EL1 is set to the same value
>> as MPAM0_EL1 rather than keeping the default value. The advantages of
>> this are that it is closer to the x86 model where the closid is globally
>> applicable, all partids are usable from user-space and user-space can't
>> bypass MPAM controls by doing the work in the kernel. However, this
>> causes some priority inversion where a high priority task waits to take
>> a mutex held by another whose resources are restricted by MPAM.
>> Thanks,
>
> In our experience, the disadvantages of the x86 model were worse
> because they triggered on hosts unintentionally, while making the
> kernel do work unrestricted on behalf of the user at least requires
> intentional abuse.
>
> -Peter

Thanks for the quick feedback. Do you have any more data/information on
this that you can share?

Thanks,

Ben