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List:       kvm
Subject:    Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding
From:       Andy Lutomirski <luto () amacapital ! net>
Date:       2020-01-31 21:33:17
Message-ID: E1F9CE39-7D61-43E1-B871-6D4BFA4B6D66 () amacapital ! net
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> On Jan 31, 2020, at 1:04 PM, Sean Christopherson <sean.j.christopherson@intel.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@intel.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.=


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