linux-kernel - Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

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Date:   Mon, 21 Oct 2019 15:02:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock

On 16/10/19 18:23, Sean Christopherson wrote:
>> Yes we can get fancy, but remember that KVM is not yet supporting
>> emulation of locked instructions.  Adding it is possible but shouldn't
>> be in the critical path for the whole feature.
> Ah, didn't realize that.  I'm surprised emulating all locks with cmpxchg
> doesn't cause problems (or am I misreading the code?).

It would cause problems if something was trying to do crazy stuff such
as locked operations on MMIO registers, or more plausibly (sort of...)
SMP in big real mode on pre-Westmere processors.  I've personally never
seen X86EMUL_CMPXCHG_FAILED happen in the real world.

Paolo

> reading the code correctly, the #AC path could kick all other vCPUS on
> emulation failure and then retry emulation to "guarantee" success.  Though
> that's starting to build quite the house of cards.
>