Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

[Xiaoyao Li]

On 11/22/2019 10:21 AM, Andy Lutomirski wrote:
> > On Nov 21, 2019, at 5:52 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > ïOn Thu, Nov 21, 2019 at 03:53:29PM -0800, Fenghua Yu wrote:
> > > > On Thu, Nov 21, 2019 at 03:18:46PM -0800, Andy Lutomirski wrote:
> > > >
> > > > > On Nov 21, 2019, at 2:29 PM, Luck, Tony <tony.luck@xxxxxxxxx> wrote:
> > > > >
> > > > > > It would be really, really nice if we could pass this feature through to a VM. Can we?
> > > > >
> > > > > It's hard because the MSR is core scoped rather than thread scoped.  So on an HT
> > > > > enabled system a pair of logical processors gets enabled/disabled together.
> > > >
> > > > Well that sucks.
> > > >
> > > > Could we pass it through if the host has no HT?  Debugging is *so* much
> > > > easier in a VM.  And HT is a bit dubious these days anyway.
> > >
> > > I think it's doable to pass it through to KVM. The difficulty is to disable
> > > split lock detection in KVM because that will disable split lock on the whole
> > > core including threads for the host. Without disabling split lock in KVM,
> > > it's doable to debug split lock in KVM.
> > >
> > > Sean and Xiaoyao are working on split lock for KVM (in separate patch set).
> > > They may have insight on how to do this.
> >
> > Yes, with SMT off KVM could allow the guest to enable split lock #AC, but
> > for the initial implementation we'd want to allow it if and only if split
> > lock #AC is disabled in the host kernel.  Otherwise we have to pull in the
> > logic to control whether or not a guest can disable split lock #AC, what
> > to do if a split lock #AC happens when it's enabled by the host but
> > disabled by the guest, etc...
>
> Whatâs the actual issue?  Thereâs a window around entry and exit when a split lock in the host might not give #AC, but as long as no user code is run, this doesnât seem like a big problem.
The problem is that guest can trigger split locked memory access just by 
disabling split lock #AC even when host has it enabled. In this 
situation, there is bus lock held on the hardware without #AC triggered, 
which is conflict with the purpose that host enables split lock #AC