linux-kernel - Re: [RFC PATCH 1/3] x86/mm: fix LAM cr3 mask inconsistency during context switch

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Date: Thu, 7 Mar 2024 15:32:53 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>,
 Andy Lutomirski <luto@...nel.org>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, x86@...nel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] x86/mm: fix LAM cr3 mask inconsistency during
 context switch

On 3/7/24 15:21, Yosry Ahmed wrote:
>>> The "set_" naming bugs me in both of the sites that get modified here.
>>> I'd be with a new name that fits better, if we can think of one.
>> Is it because it's not clear we are updating cpu_tlbstate (in which case
>> I think update_cpu_tlbstate_lam() is an improvement), or is it because
>> the function returns a value now? 

That's part of it.

>> If the latter we can put "return" in the name somewhere, or keep
>> the function void and pass in an output parameter.
No, adding a "_return" won't make it better.

> Or we can avoid returning a value from the helper and avoid passing an
> mm. The callers would be more verbose, they'll have to call
> mm_lam_cr3_mask() and mm_untag_mask() and pass the results into the
> helper (set_tlbstate_lam_mode() or update_cpu_tlbstate_lam()). Another
> advantage of this is that we can move the READ_ONCE to
> switch_mm_irqs_off() and keep the comment here.

One thing I don't like about set_tlbstate_lam_mode() is that it's not
obvious that it's writing to "cpu_tlbstate" and its right smack in the
middle of a bunch of other writers to the same thing.

But I'm also not sure that open-coding it at its three call sites makes
things better overall.

I honestly don't have any brilliant suggestions.