linux-kernel - Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ

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Message-ID: <xhsmhh64qhssj.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 19 Feb 2025 16:13:00 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Dave Hansen <dave.hansen@...el.com>, Jann Horn <jannh@...gle.com>
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Subject: Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer
 flush_tlb_kernel_range() targeting NOHZ_FULL CPUs

On 18/02/25 16:39, Dave Hansen wrote:
> On 2/18/25 14:40, Valentin Schneider wrote:
>>> 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.
>
> Having part of VMEMMAP not in the CR3 tree should be harmless while
> running userspace. VMEMMAP is a purely software structure; the hardware
> doesn't do implicit supervisor accesses to it. It's also not needed in
> super early entry.
>
> Maybe I missed part of the discussion though. Is VMEMMAP your only
> concern? I would have guessed that the more generic vmalloc()
> functionality would be harder to pin down.

Urgh, that'll teach me to send emails that late - I did indeed mean the
vmalloc() range, not at all VMEMMAP. IIUC *neither* are present in the user
kPTI page table and AFAICT the page table swap is done before the actual vmap'd
stack (CONFIG_VMAP_STACK=y) gets used.