linux-kernel - Re: [PATCH v10 12/30] x86/resctrl: Expand the width of domid by replacing mon_data

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Message-ID: <ee3567ee-584c-4994-bb7d-cb0d054e682b@intel.com>
Date: Thu, 8 May 2025 15:44:50 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: James Morse <james.morse@....com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.com>, Babu Moger
	<Babu.Moger@....com>, <shameerali.kolothum.thodi@...wei.com>, "D Scott
 Phillips OS" <scott@...amperecomputing.com>, <carl@...amperecomputing.com>,
	<lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
	<tan.shaopeng@...itsu.com>, <baolin.wang@...ux.alibaba.com>, Jamie Iles
	<quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>,
	<peternewman@...gle.com>, <dfustini@...libre.com>, <amitsinght@...vell.com>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>,
	"Dave Martin" <dave.martin@....com>, Koba Ko <kobak@...dia.com>, Shanker
 Donthineni <sdonthineni@...dia.com>, <fenghuay@...dia.com>, Tony Luck
	<tony.luck@...el.com>, Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v10 12/30] x86/resctrl: Expand the width of domid by
 replacing mon_data_bits

Hi James,

On 5/8/25 10:18 AM, James Morse wrote:
> MPAM platforms retrieve the cache-id property from the ACPI PPTT table.
> The cache-id field is 32 bits wide. Under resctrl, the cache-id becomes
> the domain-id, and is packed into the mon_data_bits union bitfield.
> The width of cache-id in this field is 14 bits.
> 
> Expanding the union would break 32bit x86 platforms as this union is
> stored as the kernfs kn->priv pointer. This saved allocating memory
> for the priv data storage.
> 
> The firmware on MPAM platforms have used the PPTT cache-id field to
> expose the interconnect's id for the cache, which is sparse and uses
> more than 14 bits. Use of this id is to enable PCIe direct cache
> injection hints. Using this feature with VFIO means the value provided
> by the ACPI table should be exposed to user-space.
> 
> To support cache-id values greater than 14 bits, convert the
> mon_data_bits union to a structure. These are shared between control
> and monitor groups, and are allocated on first use. The list of
> allocated struct mon_data is free'd when the filesystem is umount()ed.
> 
> Co-developed-by: Tony Luck <tony.luck@...el.com>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> Signed-off-by: James Morse <james.morse@....com>
> Tested-by: Babu Moger <babu.moger@....com>
> Tested-by: Shaopeng Tan <tan.shaopeng@...fujitsu.com>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette