Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit

[Xiaoyao Li]

On 7/23/2020 9:21 AM, Sean Christopherson wrote:
> On Wed, Jul 01, 2020 at 04:49:49PM +0200, Vitaly Kuznetsov wrote:
> > Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> > > So you want an exit to userspace for every bus lock and leave it all to
> > > userspace. Yes, it's doable.
> >
> > In some cases we may not even want to have a VM exit: think
> > e.g. real-time/partitioning case when even in case of bus lock we may
> > not want to add additional latency just to count such events.
>
> Hmm, I suspect this isn't all that useful for real-time cases because they'd
> probably want to prevent the split lock in the first place, e.g. would prefer
> to use the #AC variant in fatal mode.  Of course, the availability of split
> lock #AC is a whole other can of worms.
>
> But anyways, I 100% agree that this needs either an off-by-default module
> param or an opt-in per-VM capability.

Maybe on-by-default or an opt-out per-VM capability?
Turning it on introduces no overhead if no bus lock happens in guest but 
gives KVM the capability to track every potential bus lock. If user 
doesn't want the extra latency due to bus lock VM exit, it's better try 
to fix the bus lock, which also incurs high latency.

> > I'd suggest we make the new capability tri-state:
> > - disabled (no vmexit, default)
> > - stats only (what this patch does)
> > - userspace exit
> > But maybe this is an overkill, I'd like to hear what others think.
>
> Userspace exit would also be interesting for debug.  Another throttling
> option would be schedule() or cond_reched(), though that's probably getting
> into overkill territory.

We're going to leverage host's policy, i.e., calling 
handle_user_bus_lock(), for throttling, as proposed in 
https://lkml.kernel.org/r/1595021700-68460-1-git-send-email-fenghua.yu@xxxxxxxxx