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

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Message-ID: <d0450bc8-6585-49ca-9cad-49e65934bd5c@intel.com>
Date: Tue, 18 Feb 2025 16:39:34 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Valentin Schneider <vschneid@...hat.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 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.