linux-kernel - Re: [PATCH 3/3] KVM: VMX: Extend VMX's #AC interceptor to handle split lock #AC in guest

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Date:   Thu, 2 Apr 2020 13:36:08 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        x86@...nel.org, "Kenneth R . Crudup" <kenny@...ix.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Nadav Amit <namit@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Tony Luck <tony.luck@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jessica Yu <jeyu@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: VMX: Extend VMX's #AC interceptor to handle split lock #AC in guest



> On Apr 2, 2020, at 1:07 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 

> 
> 
> TBH, the more I learn about this, the more I tend to just give up on
> this whole split lock stuff in its current form and wait until HW folks
> provide something which is actually usable:
> 
>   - Per thread
>   - Properly distinguishable from a regular #AC via error code

Why the latter?  I would argue that #AC from CPL3 with EFLAGS.AC set is almost by construction not a split lock. In particular, if you meet these conditions, how exactly can you do a split lock without simultaneously triggering an alignment check?  (Maybe CMPXCHG16B?

> 
> OTOH, that means I won't be able to use it before retirement. Oh well.
> 
> Thanks,
> 
>        tglx