linux-kernel - Re: [PATCH v2 1/4] x86/resctrl: Enable non-contiguous bits in Intel CAT

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Date:   Wed, 27 Sep 2023 12:08:33 +0200
From:   Peter Newman <peternewman@...gle.com>
To:     Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>
Cc:     bp@...en8.de, dave.hansen@...ux.intel.com, fenghua.yu@...el.com,
        hpa@...or.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
        reinette.chatre@...el.com, tglx@...utronix.de, eranian@...gle.com,
        x86@...nel.org
Subject: Re: [PATCH v2 1/4] x86/resctrl: Enable non-contiguous bits in Intel CAT

Hi Maciej,

On Wed, Sep 27, 2023 at 11:20 AM Maciej Wieczór-Retman
<maciej.wieczor-retman@...el.com> wrote:
> On 2023-09-22 at 16:14:41 +0200, Peter Newman wrote:
> >On Fri, Sep 22, 2023 at 10:48:23AM +0200, Maciej Wieczor-Retman wrote:
> >> In Intel CAT CPUID.0x10.1:ECX[3] and CPUID.0x10.2:ECX[3] stopped
> >> being reserved and now carry information about non-contiguous 1s
> >> value support for L3 and L2 cache respectively. The CAT
> >> capacity bitmask (CBM) supports a non-contiguous 1s value if
> >> the bit is set.
> >
> >How new of an SDM do I need? The June 2023 revision I downloaded today didn't
> >list it.
>
> It's not currently in the SDM but in the Intel® Architecture
> Instruction Set Extensions and Future Features (which I mentioned in the
> second paragraph of the cover letter). My version of the ISA pdf was
> from June 2023.
>

I see it now, thanks!

> >> -    cpuid_count(0x00000010, idx, &eax.full, &ebx, &ecx, &edx.full);
> >> +    cpuid_count(0x00000010, idx, &eax.full, &ebx, &ecx.full, &edx.full);
> >>      hw_res->num_closid = edx.split.cos_max + 1;
> >>      r->cache.cbm_len = eax.split.cbm_len + 1;
> >>      r->default_ctrl = BIT_MASK(eax.split.cbm_len + 1) - 1;
> >>      r->cache.shareable_bits = ebx & r->default_ctrl;
> >>      r->data_width = (r->cache.cbm_len + 3) / 4;
> >> +    if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL)
> >> +            r->cache.arch_has_sparse_bitmaps = ecx.split.noncont;
> >
> >This seems to be called after the clearing of arch_has_sparse_bitmaps in
> >cache_alloc_hsw_probe(). If we can't make use of the CPUID bit on Haswell,
> >is it safe to use its value here?
>
> I believe the calls go like this for a haswell system:
> resctrl_late_init() -> check_quirks() -> __check_quirks_intel() ->
> -> cache_alloc_hsw_probe()
>
> There this line is executed:
>         rdt_alloc_capable = true;
> where rdt_alloc_capable is global in the file scope.
>
> Then later in:
> resctrl_late_init() -> get_rdt_resources() -> get_rdt_alloc_resources()
>
> this is executed at the function beginning:
>         if (rdt_alloc_capable)
>                 return true;
>
> So the rest of the get_rdt_alloc_resources() is skipped and calls to
> rdt_get_cache_alloc_cfg() never get executed.

Yuck. But it works I guess.

The series looks fine to me.

Reviewed-by: Peter Newman <peternewman@...gle.com>

I applied the series and was able to confirm the behavior was still
correct for contiguous-bitmap Intel hardware and that sprase_bitmaps
is true on AMD and continues to work as expected.

Tested-by: Peter Newman <peternewman@...gle.com>

I'm not sure if I have access to any Intel hardware with
non-contiguous bitmaps right now. Are you able to say where that would
be implemented?

Thanks!
-Peter