Re: [PATCH 0/4] x86,fs/resctrl: kernel-mode (PLZA) fixes found during review

[Babu Moger <babu.moger@amd.com>]

Hi Qinyun,


On 6/11/26 06:17, Qinyun Tan wrote:
> Hi Babu,
> 
> While reviewing this v3 series I found a few issues in the kernel-mode
> (PLZA) support and wrote a fix for each.
> I'm sending them as a small follow-up set on top of v3 so they are easy
> to fold into the next revision, or to take as separate patches --
> whichever you prefer.  The patches are ordered by dependency (build fix
> -> semantic fix -> the two binding fixes) so the series is bisectable on
> top of v3.
> 
> Patch 1 (ARM MPAM build fix): fs/resctrl now calls
> resctrl_arch_get_kmode_support()/resctrl_arch_configure_kmode(), which
> are only implemented on x86, so an aarch64 allyesconfig (MPAM) fails to
> link.  Add empty arch stubs, and hide info/kernel_mode on platforms that
> advertise no mode beyond inherit_ctrl_and_mon.
> 
> Patch 2 (RMID_EN + RDTMON_GROUP): RMID_EN is hardcoded to 1, so
> inherit_mon counts kernel-mode traffic under the PLZA RMID instead of
> inheriting from PQR_ASSOC; and assign_mon is forced to bind an
> RDTMON_GROUP, wasting an RMID.  Make RMID_EN mode-based and let assign_mon
> also accept a control group.  This is the issue we discussed earlier and
> you confirmed; this is the patch for it.
> 
> Patch 3 (atomic switch): resctrl_kernel_mode_write() releases the
> previous binding before it programs the new one.  If programming the new
> binding fails (-ENOMEM, or a pseudo-locked target group), the old,
> working binding is already gone -- a user who only tried to switch loses
> the original configuration too.  Make the switch atomic: all fallible
> work is done before the old binding is released, so a failed switch is a
> no-op.
> 
> Patch 4 (CPU online): the PLZA MSR is per-CPU and is only written over
> the CPUs that are online at bind time / mask change; nothing reprograms a
> CPU that comes online afterwards.  A hot-added vCPU, or a CPU that was
> offline at bind time, then runs with PLZA off although it is in scope,
> while info/kernel_mode still reports the binding as active.  Drive the
> per-CPU state from resctrl_online_cpu() so it is synced idempotently on
> every online (and stale enable bits are cleared for a CPU that left the
> scope while offline).
> 
> Concretely, the patch 4 failure mode is: offline a CPU, bind a
> global-assign mode while it is absent, then online it -- the onlined CPU
> is left with PLZA_EN=0 although it is in scope, while a CPU that was
> present at bind time has PLZA_EN=1, so its CPL0 traffic is not accounted
> to the bound kernel-mode group.
> 
> I'd appreciate your view on whether these match your intent for the
> design.
> 
> Qinyun Tan (4):
>    resctrl: Add kmode arch stubs for ARM MPAM and hide kernel_mode on
>      non-PLZA platforms
>    resctrl: Fix PLZA RMID_EN to be mode-based and relax RDTMON_GROUP
>      constraint for assign_mon
>    fs/resctrl: make a failed kernel-mode switch a no-op
>    fs/resctrl: program PLZA on a CPU that comes online under a binding


I have gone thru all your patches. Patches look good to me.
I also ran some basic tests to make sure it works as expected.

Note the I am still waiting for comments from maintainer (Reinette).

I will fold your changes in v4 whenever that happens.

Thanks for the review and changes.

Thanks
Babu