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

[Andy Lutomirski]

> On Nov 21, 2019, at 6:39 PM, Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
> 
> ïOn 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

Fair enough. You need some way to get this enabled in guests eventually, though.