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

Mark Rutland
2025-02-11T14:03:23Z

On Tue, Feb 11, 2025 at 02:33:51PM +0100, Valentin Schneider wrote:
> On 10/02/25 23:08, Jann Horn wrote:
> > On Mon, Feb 10, 2025 at 7:36 PM Valentin Schneider <vschneid@redhat.com> wrote:
> >> What if isolated CPUs unconditionally did a TLBi as late as possible in
> >> the stack right before returning to userspace? This would mean that upon
> >> re-entering the kernel, an isolated CPU's TLB wouldn't contain any kernel
> >> range translation - with the exception of whatever lies between the
> >> last-minute flush and the actual userspace entry, which should be feasible
> >> to vet? Then AFAICT there wouldn't be any work/flush to defer, the IPI
> >> could be entirely silenced if it targets an isolated CPU.
> >
> > Two issues with that:
>
> Firstly, thank you for entertaining the idea :-)
>
> > 1. I think the "Common not Private" feature Will Deacon referred to is
> > incompatible with this idea:
> > <https://developer.arm.com/documentation/101811/0104/Address-spaces/Common-not-Private>
> > says "When the CnP bit is set, the software promises to use the ASIDs
> > and VMIDs in the same way on all processors, which allows the TLB
> > entries that are created by one processor to be used by another"
>
> Sorry for being obtuse - I can understand inconsistent TLB states (old vs
> new translations being present in separate TLBs) due to not sending the
> flush IPI causing an issue with that, but not "flushing early". Even if TLB
> entries can be shared/accessed between CPUs, a CPU should be allowed not to
> have a shared entry in its TLB - what am I missing?
>
> > 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?

I think it's easier to ignore speculation entirely, and just assume that
the MMU can arbitrarily fill TLB entries from any page table entries
which are valid/accessible in the active page tables. Hardware
prefetchers can do that regardless of the specific path of speculative
execution.

Thus TLB fills are not limited to VAs which would be used on that
return-to-userspace path.

> Furthermore, I would hope that once a CPU is executing in userspace, it's
> not going to populate the TLB with kernel address translations - AIUI the
> whole vulnerability mitigation debacle was about preventing this sort of
> thing.

The CPU can definitely do that; the vulnerability mitigations are all
about what userspace can observe rather than what the CPU can do in the
background. Additionally, there are features like SPE and TRBE that use
kernel addresses while the CPU is executing userspace instructions.

The latest ARM Architecture Reference Manual (ARM DDI 0487 L.a) is fairly clear
about that in section D8.16 "Translation Lookaside Buff", where it says
(among other things):

When address translation is enabled, if a translation table entry
meets all of the following requirements, then that translation table
entry is permitted to be cached in a TLB or intermediate TLB caching
structure at any time:
• The translation table entry itself does not generate a Translation
fault, an Address size fault, or an Access flag fault.
• The translation table entry is not from a translation regime
configured by an Exception level that is lower than the current
Exception level.

Here "permitted to be cached in a TLB" also implies that the HW is
allowed to fetch the translation tabl entry (which is what ARM call page
table entries).

The PDF can be found at:

https://developer.arm.com/documentation/ddi0487/la/?lang=en

Mark.