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Tue, 28 Jul 2026 21:58:38 +0000 Message-ID: <6d006c4c-907a-435f-ac94-32974562ce2d@intel.com> Date: Tue, 28 Jul 2026 14:58:35 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] x86/resctrl, Documentation: Keep mbm_assign_mode "default" on boot To: Babu Moger , "Luck, Tony" , Borislav Petkov CC: "x86@kernel.org" , "Dave.Martin@arm.com" , "james.morse@arm.com" , "corbet@lwn.net" , "skhan@linuxfoundation.org" , "tglx@kernel.org" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "hpa@zytor.com" , "linux-kernel@vger.kernel.org" , "linux-doc@vger.kernel.org" , "Eranian, Stephane" , "peternewman@google.com" References: <8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger@amd.com> <20260728191818.GBamkAeh7BD1d6Gd8X@fat_crate.local> <2c098892-5d4b-402b-bdfb-59e3bf164c8d@amd.com> <73737438-01ed-437e-846a-5aa8bd687462@amd.com> <7495c998-4f3f-4ea5-a383-350584afb42c@amd.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: <7495c998-4f3f-4ea5-a383-350584afb42c@amd.com> Content-Type: text/plain; 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>>>                   if (am) >>>                           am->prev_msr = 0; >>>           } >> >> Nope. If you want the value reported to the user to start over from zero, >> then prev_msr should be set to the current value of the counter. That >> would make the next read start from zero. >> > When an RMID is reallocated, the hardware detects the state > transition and the first read returns "Unavailable". At that point, > the counter is reset and begins counting again from 0. This is the > expected behavior on AMD systems. Therefore, setting prev_msr to 0 > is the correct behavior. I believe Tony was curious how this looks from the user's perspective. >From what I understand it is neither of the presented options but instead: 3) Internal cache of hardware counter (prev_msr) is reset when resctrl discovers that the underlying counter has been re-assigned. There is a separate software maintained cumulative count to which the hardware counter value is added as the event is tracked. This cumulative count is the value returned to the user. The user thus always sees that the event count increase and it is guaranteed to only increase based on a portion of memory bandwidth consumed by the RMID/event being tracked. The returned (cumulative) count will not be accurate if the underlying counter was re-assigned. The user does not have insight into how long the underlying counter(s) was assigned to the event being counted. Reinette