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SubjectRe: x86, possible bug in __memmove() alternatives patching
On 3/26/22 04:39, Matthias Welwarsky wrote:
>
>> But, we do try to make the kernel work even the face of funky
>> hypervisors that do things that never occur on real hardware. If a nice
>> patch to fix this up showed up, I'd definitely take a look.
> The question is whether a sequence like this could be relevant:
>
> 0) CPU announces feature FSRM through cpuid
> 1) BIOS/firmware disables fast string ops through IA32_MISC_ENABLE before
> loading kernel (for whatever reason)
> 2) Kernel populates features from cpuid
> 3) Kernel clears ERMS based on IA32_MISC_ENABLE
> 4) "alternatives" patching destroys __memmove()

Hi Matthias,

What does "destroys __memmove()" mean in practice? What's the end-user
visible effect of this? Do they see a crash or just crummy performance?

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