linux-kernel - Re: [PATCH 4/4] x86/cpu: Use SERIALIZE in sync

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Date:   Tue, 28 Jul 2020 10:50:49 +0200
From:   peterz@...radead.org
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     hpa@...or.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Cathy Zhang <cathy.zhang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Kyung Min Park <kyung.min.park@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-kernel@...r.kernel.org,
        Ricardo Neri <ricardo.neri@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-edac@...r.kernel.org, frederic@...nel.org
Subject: Re: [PATCH 4/4] x86/cpu: Use SERIALIZE in sync_core() when available

On Mon, Jul 27, 2020 at 09:41:14PM -0700, Ricardo Neri wrote:
> I think I got a little lost here.

Hehe, sorry. I got carried away, it's just that recently people
expressed interest in 'fixing' some of the text_poke_sync() issues
again.

> If I understand correctly, there are
> two alternatives to implement support for serialize better:
> 
>   a) alternative(IRET_TO_SELF, SERIALIZE, X86_FEATURE_SERIALIZE); or
>   b) asm volatile("1:.byte 0xf, 0x1, 0xe8;2:" _ASM_EXTABLE(1b:2b)
> 
> a) would be the traditional and simpler solution. b) would rely on
> causing an #UD and getting an IRET on existing hardware b) would need some
> more optimization work when handling the exception and a few reworks on
> the poke patching code.
> 
> Which option should I focus on? Which option would be more desirable/better?

I'd say go with (a) for now. We can always go overboard later ;-)