Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs

Valentin Schneider
2025-02-18T22:40:39Z

On 11/02/25 06:22, Dave Hansen wrote:
> On 2/11/25 05:33, Valentin Schneider wrote:
>>> 2. It's wrong to assume that TLB entries are only populated for
>>> addresses you access - thanks to speculative execution, you have to
>>> assume that the CPU might be populating random TLB entries all over
>>> the place.
>> Gotta love speculation. Now it is supposed to be limited to genuinely
>> accessible data & code, right? Say theoretically we have a full TLBi as
>> literally the last thing before doing the return-to-userspace, speculation
>> should be limited to executing maybe bits of the return-from-userspace
>> code?
>
> In practice, it's mostly limited like that.
>
> Architecturally, there are no promises from the CPU. It is within its
> rights to cache anything from the page tables at any time. If it's in
> the CR3 tree, it's fair game.
>

So what if the VMEMMAP range *isn't* in the CR3 tree when a CPU is
executing in userspace?

AIUI that's the case with kPTI - the remaining kernel pages should mostly
be .entry.text and cpu_entry_area, at least for x86.

It sounds like it wouldn't do much for arm64 though, if with CnP a CPU executing in
userspace and with the user/trampoline page table installed can still use
TLB entries of another CPU executing in kernelspace with the kernel page
table installed.