linux-kernel - Re: [External] [PATCH RFC v2 03/17] RISC-V: Add support for srmcfg CSR from Ssqosid ext

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Message-ID: <CAEEQ3wntQWi_itwA716qjaFBMd18FR3nb6rMz5STw+DGtYjp+A@mail.gmail.com>
Date: Mon, 9 Feb 2026 11:36:16 +0800
From: yunhui cui <cuiyunhui@...edance.com>
To: Drew Fustini <fustini@...nel.org>
Cc: Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>, 
	Radim Krčmář <rkrcmar@...tanamicro.com>, 
	Samuel Holland <samuel.holland@...ive.com>, Adrien Ricciardi <aricciardi@...libre.com>, 
	Nicolas Pitre <npitre@...libre.com>, Kornel Dulęba <mindal@...ihalf.com>, 
	Atish Patra <atish.patra@...ux.dev>, Atish Kumar Patra <atishp@...osinc.com>, 
	Vasudevan Srinivasan <vasu@...osinc.com>, Ved Shanbhogue <ved@...osinc.com>, 
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	Reinette Chatre <reinette.chatre@...el.com>, Tony Luck <tony.luck@...el.com>, 
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	linux-riscv@...ts.infradead.org, x86@...nel.org, 
	Rob Herring <robh@...nel.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>, 
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	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Paul Walmsley <paul.walmsley@...ive.com>, linux-acpi@...r.kernel.org, 
	acpica-devel@...ts.linux.dev, devicetree@...r.kernel.org
Subject: Re: [External] [PATCH RFC v2 03/17] RISC-V: Add support for srmcfg
 CSR from Ssqosid ext

Hi Drew,

On Sun, Feb 8, 2026 at 9:31 AM Drew Fustini <fustini@...nel.org> wrote:
>
> On Mon, Feb 02, 2026 at 11:17:52AM +0800, yunhui cui wrote:
> > Hi Drew,
> >
> > On Thu, Jan 29, 2026 at 4:28 AM Drew Fustini <fustini@...nel.org> wrote:
> > >
> > > Add support for the srmcfg CSR defined in the Ssqosid ISA extension
> > > (Supervisor-mode Quality of Service ID). The CSR contains two fields:
> > >
> > >   - Resource Control ID (RCID) used determine resource allocation
> > >   - Monitoring Counter ID (MCID) used to track resource usage
> > >
> > > Requests from a hart to shared resources like cache will be tagged with
> > > these IDs. This allows the usage of shared resources to be associated
> > > with the task currently running on the hart.
> > >
> > > A srmcfg field is added to thread_struct and has the same format as the
> > > srmcfg CSR. This allows the scheduler to set the hart's srmcfg CSR to
> > > contain the RCID and MCID for the task that is being scheduled in. The
> > > srmcfg CSR is only written to if the thread_struct.srmcfg is different
> > > than the current value of the CSR.
> > >
> > > A per-cpu variable cpu_srmcfg is used to mirror that state of the CSR.
> > > This is because access to L1D hot memory should be several times faster
> > > than a CSR read. Also, in the case of virtualization, accesses to this
> > > CSR are trapped in the hypervisor.
> > >
> > > Link: https://github.com/riscv/riscv-ssqosid/releases/tag/v1.0
> > > Co-developed-by: Kornel Dulęba <mindal@...ihalf.com>
> > > Signed-off-by: Kornel Dulęba <mindal@...ihalf.com>
> > > [fustini: rename csr, refactor switch_to, rebase on upstream]
> > > Signed-off-by: Drew Fustini <fustini@...nel.org>
> [..]
> > > diff --git a/arch/riscv/include/asm/qos.h b/arch/riscv/include/asm/qos.h
> > > new file mode 100644
> > > index 000000000000..84830d7c6dc4
> > > --- /dev/null
> > > +++ b/arch/riscv/include/asm/qos.h
> > > @@ -0,0 +1,41 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +#ifndef _ASM_RISCV_QOS_H
> > > +#define _ASM_RISCV_QOS_H
> > > +
> > > +#ifdef CONFIG_RISCV_ISA_SSQOSID
> > > +
> > > +#include <linux/sched.h>
> > > +#include <linux/jump_label.h>
> > > +
> > > +#include <asm/barrier.h>
> > > +#include <asm/csr.h>
> > > +#include <asm/hwcap.h>
> > > +
> > > +/* cached value of srmcfg csr for each cpu */
> > > +DECLARE_PER_CPU(u32, cpu_srmcfg);
> > > +
> > > +static inline void __switch_to_srmcfg(struct task_struct *next)
> > > +{
> > > +       u32 *cpu_srmcfg_ptr = this_cpu_ptr(&cpu_srmcfg);
> > > +       u32 thread_srmcfg;
> > > +
> > > +       thread_srmcfg = READ_ONCE(next->thread.srmcfg);
> >
> >
> > First set the cpu_list, and then the condition thread_srmcfg !=
> > *cpu_srmcfg_ptr will not be satisfied. Is a default value required
> > here? Both code paths for cpu_list and tasks are compared against the
> > default value; you may refer to the implementation of mpam.
>
> I'm having trouble finding cpu_list but I think that it does make sense
> to set the initial value.
>
> Were you thinking I should look at mpam_set_cpu_defaults() in
> the mpam_resctrl_glue_v4 [1] branch?

To be exact, it's "cpus_list".  yep, we need logic similar to that of
arm64_mpam_default.

>
> Thanks,
> Drew
>
> [1] https://gitlab.arm.com/linux-arm/linux-bh.git

Thanks,
Yunhui