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

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Date:   Fri, 31 Jan 2020 13:04:24 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
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 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.