linux-kernel - Re: [PATCH v5 00/34] sparc64: vdso: Switch to the generic vDSO library

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Message-Id: <7eb1a661-e5bf-45cb-a3cf-7337fb0c4173@app.fastmail.com>
Date: Sun, 09 Nov 2025 17:08:27 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Maciej W. Rozycki" <macro@...am.me.uk>
Cc: "Andy Lutomirski" <luto@...nel.org>,
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
 "Thomas Gleixner" <tglx@...utronix.de>,
 "Vincenzo Frascino" <vincenzo.frascino@....com>,
 "David S . Miller" <davem@...emloft.net>,
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 loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
 linux-s390@...r.kernel.org, "Arnd Bergmann" <arnd@...nel.org>
Subject: Re: [PATCH v5 00/34] sparc64: vdso: Switch to the generic vDSO library

On Sun, Nov 9, 2025, at 04:23, Maciej W. Rozycki wrote:
> On Sat, 8 Nov 2025, Arnd Bergmann wrote:
>
>> On other architectures, I see that parisc (always aliasing) has stubbed
>> out the vdso functions, while mips/loongson has limited the page size
>> selection to never alias. A few other mips platforms can theoretically
>> enable both small pages and vdso, but my guess is that in practice
>> they don't use the vdso (mips32/ath79) or they use 16KB pages
>> (rm, dec, ip22) based on the defconfig settings.
>
>  Umm, I'd have to dive into the details (and I hardly have the resources 
> at hand), but quite a bunch of MIPS microarchitectures suffer from cache 
> aliases; some even have VIVT caches.

I was going with this list:

$ git grep define.cpu_has_dc_aliases arch/mips/
arch/mips/include/asm/cpu-features.h:#define cpu_has_dc_aliases (cpu_data[0].dcache.flags & MIPS_CACHE_ALIASES)
arch/mips/include/asm/mach-ath79/cpu-feature-overrides.h:#define cpu_has_dc_aliases     1
arch/mips/include/asm/mach-au1x00/cpu-feature-overrides.h:#define cpu_has_dc_aliases            0
arch/mips/include/asm/mach-bcm63xx/cpu-feature-overrides.h:#define cpu_has_dc_aliases           0
arch/mips/include/asm/mach-cavium-octeon/cpu-feature-overrides.h:#define cpu_has_dc_aliases     0
arch/mips/include/asm/mach-dec/cpu-feature-overrides.h:#define cpu_has_dc_aliases               0
arch/mips/include/asm/mach-dec/cpu-feature-overrides.h:#define cpu_has_dc_aliases               (PAGE_SIZE < 0x4000)
arch/mips/include/asm/mach-ingenic/cpu-feature-overrides.h:#define cpu_has_dc_aliases   0
arch/mips/include/asm/mach-ip22/cpu-feature-overrides.h:#define cpu_has_dc_aliases      (PAGE_SIZE < 0x4000)
arch/mips/include/asm/mach-ip27/cpu-feature-overrides.h:#define cpu_has_dc_aliases              0
arch/mips/include/asm/mach-ip28/cpu-feature-overrides.h:#define cpu_has_dc_aliases      0 /* see probe_pcache() */
arch/mips/include/asm/mach-ip30/cpu-feature-overrides.h:#define cpu_has_dc_aliases              0
arch/mips/include/asm/mach-loongson2ef/cpu-feature-overrides.h:#define cpu_has_dc_aliases       (PAGE_SIZE < 0x4000)
arch/mips/include/asm/mach-loongson64/cpu-feature-overrides.h:#define cpu_has_dc_aliases        (PAGE_SIZE < 0x4000)
arch/mips/include/asm/mach-malta/cpu-feature-overrides.h:/* #define cpu_has_dc_aliases  ? */
arch/mips/include/asm/mach-malta/cpu-feature-overrides.h:/* #define cpu_has_dc_aliases  ? */
arch/mips/include/asm/mach-ralink/mt7621/cpu-feature-overrides.h:#define cpu_has_dc_aliases     0
arch/mips/include/asm/mach-rm/cpu-feature-overrides.h:#define cpu_has_dc_aliases        (PAGE_SIZE < 0x4000)
arch/mips/include/asm/mach-sibyte/cpu-feature-overrides.h:#define cpu_has_dc_aliases    0

which for many platforms seems to come up with a compile-time
constant value. I hadn't checked the exact conditions for
the runtime MIPS_CACHE_ALIASES flag, but I see that the kernel
has a fairly centralized detection function in probe_pcache()
https://elixir.bootlin.com/linux/v6.17.7/source/arch/mips/mm/c-r4k.c#L1251

which is used for the 'generic' platform and those that don't set
a compile-time constant (ath25, bcm47xx, bmips, cobalt, ip32, malta,
pic32, rc32434 and tx49xx).

> (see the figures at the bottom; uptime quoted for an idea of the rate, 
> though the system hasn't been heavily loaded).  It is possible with the 
> aid of S$, which is inclusive and PIPT.

I now found commit 0f02cfbc3d9e ("MIPS: VDSO: Match data page
cache colouring when D$ aliases"), which probably does everything
necessary to just make it work reliably on mips, and should
be portable to sparc as Andy suggested as well.

     Arnd