Hello!
From: Dave Hansen dave.hansen@linux.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.
- https://raw.githubusercontent.com/InstLatx64/InstLatx64/master/GenuineIntel/...
- You can also get this behavior if you use "-cpu 486,+clzero" in QEMU.
Tested-by: Jörn Heusipp osmanx@heusipp.de
See https://lore.kernel.org/lkml/5e31cad3-ad4d-493e-ab07-724cfbfaba44@heusipp.de...
Best regards, Jörn