linux-kernel - Re: [PATCH RESEND] x86/entry: Don't write to CR3 when restoring to kernel CR3

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Date:   Mon, 18 Sep 2023 09:35:19 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Brendan Jackman <jackmanb@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     luto@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        laijs@...ux.alibaba.com, yosryahmed@...gle.com, reijiw@...gle.com,
        oweisse@...gle.com
Subject: Re: [PATCH RESEND] x86/entry: Don't write to CR3 when restoring to
 kernel CR3


* Brendan Jackman <jackmanb@...gle.com> wrote:

> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> Skip resuming KERNEL pages since it is already KERNEL CR3
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
> 
> While staring at paranoid_exit I was confused about why we had this CR3
> write, avoiding it seems like a free optimisation. The original commit
> 21e94459110252 ("x86/mm: Optimize RESTORE_CR3") says "Most NMI/paranoid
> exceptions will not in fact change pagetables" but I didn't't understand
> what the "most" was referring to. I then discovered this patch on the
> mailing list, Andy said[1] that it looks correct so maybe now is the
> time to merge it?
> 
> Note there's another patch in [1] as well, the benefit of that one is
> not obvious to me though.
> 
> We've tested an equivalent patch in our internal kernel.
> 
> [1] https://lore.kernel.org/lkml/20200526043507.51977-3-laijs@linux.alibaba.com/
> -- >8 --
>  arch/x86/entry/calling.h | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)

I concur that this is a good change, but it would be really nice to get an 
ack from Andy or Thomas as well.

Thanks,

	Ingo