On Mon, Oct 20, 2025 at 10:04:05AM -0500, Babu Moger wrote:
Users can create as many monitoring groups as the number of RMIDs supported by the hardware. However, on AMD systems, only a limited number of RMIDs are guaranteed to be actively tracked by the hardware. RMIDs that exceed this limit are placed in an "Unavailable" state.
When a bandwidth counter is read for such an RMID, the hardware sets MSR_IA32_QM_CTR.Unavailable (bit 62). When such an RMID starts being tracked again the hardware counter is reset to zero. MSR_IA32_QM_CTR.Unavailable remains set on first read after tracking re-starts and is clear on all subsequent reads as long as the RMID is tracked.
resctrl miscounts the bandwidth events after an RMID transitions from the "Unavailable" state back to being tracked. This happens because when the hardware starts counting again after resetting the counter to zero, resctrl in turn compares the new count against the counter value stored from the previous time the RMID was tracked.
This results in resctrl computing an event value that is either undercounting (when new counter is more than stored counter) or a mistaken overflow (when new counter is less than stored counter).
Reset the stored value (arch_mbm_state::prev_msr) of MSR_IA32_QM_CTR to zero whenever the RMID is in the "Unavailable" state to ensure accurate counting after the RMID resets to zero when it starts to be tracked again.
Example scenario that results in mistaken overflow
The resctrl filesystem is mounted, and a task is assigned to a monitoring group.
$mount -t resctrl resctrl /sys/fs/resctrl $mkdir /sys/fs/resctrl/mon_groups/test1/ $echo 1234 > /sys/fs/resctrl/mon_groups/test1/tasks
$cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes 21323 <- Total bytes on domain 0 "Unavailable" <- Total bytes on domain 1
Task is running on domain 0. Counter on domain 1 is "Unavailable".
The task runs on domain 0 for a while and then moves to domain 1. The counter starts incrementing on domain 1.
$cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes 7345357 <- Total bytes on domain 0 4545 <- Total bytes on domain 1
At some point, the RMID in domain 0 transitions to the "Unavailable" state because the task is no longer executing in that domain.
$cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes "Unavailable" <- Total bytes on domain 0 434341 <- Total bytes on domain 1
Since the task continues to migrate between domains, it may eventually return to domain 0.
$cat /sys/fs/resctrl/mon_groups/test1/mon_data/mon_L3_*/mbm_total_bytes 17592178699059 <- Overflow on domain 0 3232332 <- Total bytes on domain 1
In this case, the RMID on domain 0 transitions from "Unavailable" state to active state. The hardware sets MSR_IA32_QM_CTR.Unavailable (bit 62) when the counter is read and begins tracking the RMID counting from 0.
Subsequent reads succeed but return a value smaller than the previously saved MSR value (7345357). Consequently, the resctrl's overflow logic is triggered, it compares the previous value (7345357) with the new, smaller value and incorrectly interprets this as a counter overflow, adding a large delta.
In reality, this is a false positive: the counter did not overflow but was simply reset when the RMID transitioned from "Unavailable" back to active state.
Here is the text from APM [1] available from [2].
"In PQOS Version 2.0 or higher, the MBM hardware will set the U bit on the first QM_CTR read when it begins tracking an RMID that it was not previously tracking. The U bit will be zero for all subsequent reads from that RMID while it is still tracked by the hardware. Therefore, a QM_CTR read with the U bit set when that RMID is in use by a processor can be considered 0 when calculating the difference with a subsequent read."
[1] AMD64 Architecture Programmer's Manual Volume 2: System Programming Publication # 24593 Revision 3.41 section 19.3.3 Monitoring L3 Memory Bandwidth (MBM).
[ bp: Split commit message into smaller paragraph chunks for better consumption. ]
Fixes: 4d05bf71f157d ("x86/resctrl: Introduce AMD QOS feature") Signed-off-by: Babu Moger babu.moger@amd.com Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Reviewed-by: Reinette Chatre reinette.chatre@intel.com Tested-by: Reinette Chatre reinette.chatre@intel.com Cc: stable@vger.kernel.org # needs adjustments for <= v6.17 Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 # [2] (cherry picked from commit 15292f1b4c55a3a7c940dbcb6cb8793871ed3d92)
This wasn't a "clean" cherry-pick at all, please document the changes made from the upstream version here.
thanks,
greg k-h