linux-kernel - Re: [PATCH] x86/cpu: Provide default cache line size if not enumerated

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Date: Thu, 30 May 2024 11:47:52 +0200
From: Jörn Heusipp <osmanx@...sipp.de>
To: dave.hansen@...ux.intel.com
Cc: andriy.shevchenko@...ux.intel.com, bp@...en8.de,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org, tglx@...utronix.de,
 x86@...nel.org
Subject: Re: [PATCH] x86/cpu: Provide default cache line size if not
 enumerated


Hello!

> From: Dave Hansen <dave.hansen@...ux.intel.com>
>
> tl;dr: CPUs with CPUID.80000008H but without CPUID.01H:EDX[CLFSH]
> will end up reporting cache_line_size()==0 and bad things happen.
> Fill in a default on those to avoid the problem.
>
> Long Story:
>
> The kernel dies a horrible death if c->x86_cache_alignment (aka.
> cache_line_size() is 0.  Normally, this value is populated from
> c->x86_clflush_size.
>
> Right now the code is set up to get c->x86_clflush_size from two
> places.  First, modern CPUs get it from CPUID.  Old CPUs that don't
> have leaf 0x80000008 (or CPUID at all) just get some sane defaults
> from the kernel in get_cpu_address_sizes().
>
> The vast majority of CPUs that have leaf 0x80000008 also get
> ->x86_clflush_size from CPUID.  But there are oddballs.
>
> Intel Quark CPUs[1] and others[2] have leaf 0x80000008 but don't set
> CPUID.01H:EDX[CLFSH], so they skip over filling in ->x86_clflush_size:
>
> 	cpuid(0x00000001, &tfms, &misc, &junk, &cap0);
> 	if (cap0 & (1<<19))
> 		c->x86_clflush_size = ((misc >> 8) & 0xff) * 8;
>
> So they: land in get_cpu_address_sizes(), set vp_bits_from_cpuid=0 and
> never fill in c->x86_clflush_size, assign c->x86_cache_alignment, and
> hilarity ensues in code like:
>
>         buffer = kzalloc(ALIGN(sizeof(*buffer), cache_line_size()),
>                          GFP_KERNEL);
>
> To fix this, always provide a sane value for ->x86_clflush_size.
>
> Big thanks to Andy Shevchenko for finding and reporting this and also
> providing a first pass at a fix. But his fix was only partial and only
> worked on the Quark CPUs.  It would not, for instance, have worked on
> the QEMU config.
>
> 1. https://raw.githubusercontent.com/InstLatx64/InstLatx64/master/GenuineIntel/GenuineIntel0000590_Clanton_03_CPUID.txt
> 2. You can also get this behavior if you use "-cpu 486,+clzero"
>    in QEMU.

Tested-by: Jörn Heusipp <osmanx@...sipp.de>

See
https://lore.kernel.org/lkml/5e31cad3-ad4d-493e-ab07-724cfbfaba44@heusipp.de/


Best regards,
Jörn