RE: [PATCH v4 03/13] x86/cpufeatures: Add Slow Memory Bandwidth Allocation feature flag

Moger, Babu
2022-09-16T19:02:33Z

[AMD Official Use Only - General]

Hi Reinette,

> -----Original Message-----
> From: Reinette Chatre <reinette.chatre@intel.com>
> Sent: Friday, September 16, 2022 10:54 AM
> To: Moger, Babu <Babu.Moger@amd.com>; corbet@lwn.net;
> tglx@linutronix.de; mingo@redhat.com; bp@alien8.de
> Cc: fenghua.yu@intel.com; dave.hansen@linux.intel.com; x86@kernel.org;
> hpa@zytor.com; paulmck@kernel.org; akpm@linux-foundation.org;
> quic_neeraju@quicinc.com; rdunlap@infradead.org;
> damien.lemoal@opensource.wdc.com; songmuchun@bytedance.com;
> peterz@infradead.org; jpoimboe@kernel.org; pbonzini@redhat.com;
> chang.seok.bae@intel.com; pawan.kumar.gupta@linux.intel.com;
> jmattson@google.com; daniel.sneddon@linux.intel.com; Das1, Sandipan
> <Sandipan.Das@amd.com>; tony.luck@intel.com; james.morse@arm.com;
> linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org;
> bagasdotme@gmail.com; eranian@google.com
> Subject: Re: [PATCH v4 03/13] x86/cpufeatures: Add Slow Memory Bandwidth
> Allocation feature flag
>
> Hi Babu,
>
> On 9/7/2022 11:00 AM, Babu Moger wrote:
> > Adds the new AMD feature X86_FEATURE_SMBA. With this feature, the QOS
>
> Adds -> Add

Sure

>
> > enforcement policies can be applied to external slow memory connected
> > to the host. QOS enforcement is accomplished by assigning a Class Of
> > Service (COS) to a processor and specifying allocations or limits for
> > that COS for each resource to be allocated.
> >
> > This feature is identified by the CPUID Function 8000_0020_EBX_x0.
> >
> > CPUID Fn8000_0020_EBX_x0 AMD Bandwidth Enforcement Feature
> Identifiers (ECX=0)
> > Bits Field Name Description
> > 2 L3SBE L3 external slow memory bandwidth enforcement
> >
> > Currently, CXL.memory is the only supported "slow" memory device. With
> > the support of SMBA feature the hardware enables bandwidth allocation
> > on the slow memory devices. If there are multiple slow memory devices
> > in the system, then the throttling logic groups all the slow sources
> > together and applies the limit on them as a whole.
>
> The above is a useful addition (made in patch 13/13) to the documentation ...
>
> >
> > The presence of the SMBA feature(with CXL.memory) is independent of
> > whether slow memory device is actually present in the system. If there
> > is no slow memory in the system, then setting a SMBA limit will have
> > no impact on the performance of the system.
>
> ... could the above snippet also please be added to the documentation?
>

Ok Sure.

> >
> > Presence of CXL memory can be identified by numactl command.
> >
> > $numactl -H
> > available: 2 nodes (0-1)
> > node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 node 0 size:
> > 63678 MB node 0 free: 59542 MB node 1 cpus:
> > node 1 size: 16122 MB
> > node 1 free: 15627 MB
> > node distances:
> > node 0 1
> > 0: 10 50
> > 1: 50 10
> >
> > CPU list for CXL memory will be emply. The cpu-cxl node distance is
> > greater
>
> emply -> empty?

ok
>
> > than cpu-to-cpu distances. Node 1 has the CXL memory in this case. CXL
> > memory can also be identified using ACPI SRAT table and memory maps.
> >
> > Feature description is available in the specification, "AMD64
> > Technology Platform Quality of Service Extensions, Revision: 1.03 Publication
> # 56375 Revision: 1.03 Issue Date: February 2022".
>
> Please shorten these lines to the recommended 75 chars per line.

Sure
>
> >
> > Link:
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.
> > amd.com%2Fen%2Fsupport%2Ftech-docs%2Famd64-technology-platform-
> quality
> > -service-
> extensions&amp;data=05%7C01%7Cbabu.moger%40amd.com%7C60553f32
> >
> e9ab4ed1219c08da97fbc048%7C3dd8961fe4884e608e11a82d994e183d%7C0%
> 7C0%7C
> >
> 637989404770663129%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMD
> AiLCJQIjo
> >
> iV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdat
> a=WY4H
> > EzWHdMpMUUR%2FBnBupwdHuQ6O2RfdrfcGx4TkXfI%3D&amp;reserved=0
> > Link:
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugz
> >
> illa.kernel.org%2Fshow_bug.cgi%3Fid%3D206537&amp;data=05%7C01%7Cbab
> u.m
> >
> oger%40amd.com%7C60553f32e9ab4ed1219c08da97fbc048%7C3dd8961fe488
> 4e608e
> >
> 11a82d994e183d%7C0%7C0%7C637989404770663129%7CUnknown%7CTWFpb
> GZsb3d8ey
> >
> JWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7
> C300
> >
> 0%7C%7C%7C&amp;sdata=7e7sGH0iaEW8mlst5mK3fn9wy%2FYRhDU%2BbBm
> PWzSrGL4%3
> > D&amp;reserved=0
> > Signed-off-by: Babu Moger <babu.moger@amd.com>
> > ---
> > arch/x86/include/asm/cpufeatures.h | 1 +
> > arch/x86/kernel/cpu/scattered.c | 1 +
> > 2 files changed, 2 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/cpufeatures.h
> > b/arch/x86/include/asm/cpufeatures.h
> > index 235dc85c91c3..1815435c9c88 100644
> > --- a/arch/x86/include/asm/cpufeatures.h
> > +++ b/arch/x86/include/asm/cpufeatures.h
> > @@ -304,6 +304,7 @@
> > #define X86_FEATURE_UNRET (11*32+15) /* "" AMD BTB
> untrain return */
> > #define X86_FEATURE_USE_IBPB_FW (11*32+16) /* "" Use IBPB
> during runtime firmware calls */
> > #define X86_FEATURE_RSB_VMEXIT_LITE (11*32+17) /* "" Fill RSB on
> VM exit when EIBRS is enabled */
> > +#define X86_FEATURE_SMBA (11*32+18) /* SLOW Memory
> Bandwidth Allocation */
>
> Why is "SLOW" in all caps?

Will correct it.

>
> >
> > /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
> > #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI
> instructions */
> > diff --git a/arch/x86/kernel/cpu/scattered.c
> > b/arch/x86/kernel/cpu/scattered.c index fd44b54c90d5..885ecf46abb2
> > 100644
> > --- a/arch/x86/kernel/cpu/scattered.c
> > +++ b/arch/x86/kernel/cpu/scattered.c
> > @@ -44,6 +44,7 @@ static const struct cpuid_bit cpuid_bits[] = {
> > { X86_FEATURE_CPB, CPUID_EDX, 9, 0x80000007, 0 },
> > { X86_FEATURE_PROC_FEEDBACK, CPUID_EDX, 11, 0x80000007, 0 },
> > { X86_FEATURE_MBA, CPUID_EBX, 6, 0x80000008, 0 },
> > + { X86_FEATURE_SMBA, CPUID_EBX, 2, 0x80000020, 0 },
> > { X86_FEATURE_PERFMON_V2, CPUID_EAX, 0, 0x80000022, 0 },
> > { 0, 0, 0, 0, 0 }
> > };
> >
> >
>
> Could you please follow the coding style (wrt tabs vs. spaces) of the area you
> are contributing to here? Please do so in all patches in this series.

Sure. Thanks
Babu Moger