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Tue, 14 Jul 2026 21:37:49 +0000 Message-ID: <4fabac43-f054-4285-89e8-c100310735fa@intel.com> Date: Tue, 14 Jul 2026 14:37:47 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] mpam,x86,fs/resctrl: Generic schema description Proof of Concept To: "Chen, Yu C" CC: Borislav Petkov , Thomas Gleixner , "Dave Hansen" , Peter Newman , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , Ben Horgan , Tony Luck , Dave Martin , James Morse , Drew Fustini , Babu Moger , Fenghua Yu , References: <29c95b69-e1a4-46b1-ab8b-45c09308b924@arm.com> <7f2d0cc3-266c-4fa3-a9ba-7352325c76d1@intel.com> <3ef279c9-c4b3-48cb-8235-da8a871b54f9@arm.com> <5cffdb88-924e-43e7-84f2-ea19d82910fb@intel.com> <0c957e0a-0188-40c0-96e6-4e5d787a8ae6@arm.com> <8bd94a5f-1460-4bc3-a2b2-68a298200ad8@intel.com> <36213825-c353-46b4-8a05-83ea446201a4@intel.com> <2a06bfde-2740-4bde-8ffb-aa2026d6edd1@intel.com> <6f7a02ec-03fc-4c9c-af86-239f98b9b903@intel.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <6f7a02ec-03fc-4c9c-af86-239f98b9b903@intel.com> Content-Type: text/plain; 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Is this >>>>      accurate? What will behavior be on an ERDT system that does not support >>>>      MSR access with the percentage-based control? >>> >>> Would it make sense to encourage users to switch to MMIO-based access if the >>> platform replaces MSR interfaces with MMIO-based ones? One issue with emulating >>> MSR-based access is that there may not be a strict 1:1 mapping between the MSR >>> range and the MMIO range. Additionally, I am unsure whether both are strictly >>> linearly scalable, so it could be challenging to create a proper mapping for >>> msr_value_emulated = map(mmio_value) IIUC. >> >> resctrl needs to continue to offer the MB percentage based control to not break any >> existing tools that may take a while to transition to the new control interface. >> >> I believe all architectures will encourage their users to use the finer grained >> controls while I also expect users and their tools would gradually switch by themselves >> to benefit from the improved capabilities. >> > > The following are current thoughts on how Control B can emulate Control A. > May I know if this direction is acceptable? > > The main idea is that, Legacy MBA controller delegates to region-aware MBA > controller via a pointer, with its hw_update function converting percentages > to the target's value range. > > struct resctrl_hw_ctrl { >     struct resctrl_ctrl  r_ctrl; >     unsigned int         msr_base; >     void                 (*hw_update)(struct hw_param *m); >     /* points to the backing controller */ >     struct resctrl_ctrl  *r_ctrl_emul;  <-- newly added > }; > > On boot, region-aware controllers shall be created first, followed by the > legacy MBA controller. When ERDT is enabled for legacy MBA, hw_update is > assigned to mba_emul_update() instead of mba_wrmsr_intel(). Meanwhile, > r_ctrl_emul points to the MAX controller of region0. To maintain the generic "memory bandwidth allocation" I was expecting legacy MBA to be emulated by MAX controllers of all regions, not just one. Could you please elaborate how emulating with just one region is preferred? > > In mba_emul_update(), the legacy MSR value is scaled to the range of region0 > MAX controller by referencing r_ctrl_emul->membw.max_bw and r_ctrl_emul->membw.min_bw. > This maps the original input range [1, 100] to a new range [1, L], where L <= 255. > > This emulation logic can also be extended to other controllers. > > thanks, > Chenyu Reinette