linux-kernel - Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding

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Message-Id: <E1F9CE39-7D61-43E1-B871-6D4BFA4B6D66@amacapital.net>
Date:   Fri, 31 Jan 2020 13:33:17 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding



> On Jan 31, 2020, at 1:04 PM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> 
> On Fri, Jan 31, 2020 at 12:57:51PM -0800, Andy Lutomirski wrote:
>> 
>>>> On Jan 31, 2020, at 12:18 PM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
>>> 
>>> This is essentially what I proposed a while back.  KVM would allow enabling
>>> split-lock #AC in the guest if and only if SMT is disabled or the enable bit
>>> is per-thread, *or* the host is in "warn" mode (can live with split-lock #AC
>>> being randomly disabled/enabled) and userspace has communicated to KVM that
>>> it is pinning vCPUs.
>> 
>> How about covering the actual sensible case: host is set to fatal?  In this
>> mode, the guest gets split lock detection whether it wants it or not. How do
>> we communicate this to the guest?
> 
> KVM doesn't advertise split-lock #AC to the guest and returns -EFAULT to the
> userspace VMM if the guest triggers a split-lock #AC.
> 
> Effectively the same behavior as any other userspace process, just that KVM
> explicitly returns -EFAULT instead of the process getting a SIGBUS.


Which helps how if the guest is actually SLD-aware?

I suppose we could make the argument that, if an SLD-aware guest gets #AC at CPL0, it’s a bug, but it still seems rather nicer to forward the #AC to the guest instead of summarily killing it.

ISTM, on an SLD-fatal host with an SLD-aware guest, the host should tell the guest “hey, you may not do split locks — SLD is forced on” and the guest should somehow acknowledge it so that it sees the architectural behavior instead of something we made up.  Hence my suggestion.