Re: [PATCH 3/3] KVM SVM: Add Bus Lock Detect support

[Ravi Bangoria]

Sean,

Apologies for the delay. I was waiting for Bus Lock Threshold patches to be
posted upstream:

  https://lore.kernel.org/r/20240709175145.9986-1-manali.shukla@xxxxxxx

On 12-Jun-24 7:12 AM, Sean Christopherson wrote:
> On Wed, Jun 05, 2024, Ravi Bangoria wrote:
>> On 6/5/2024 8:38 PM, Sean Christopherson wrote:
>>> Some of the problems on Intel were due to the awful FMS-based feature detection,
>>> but those weren't the only hiccups.  E.g. IIRC, we never sorted out what should
>>> happen if both the host and guest want bus-lock #DBs.
>>
>> I've to check about vcpu->guest_debug part, but keeping that aside, host and
>> guest can use Bus Lock Detect in parallel because, DEBUG_CTL MSR and DR6
>> register are save/restored in VMCB, hardware cause a VMEXIT_EXCEPTION_1 for
>> guest #DB(when intercepted) and hardware raises #DB on host when it's for the
>> host.
> 
> I'm talking about the case where the host wants to do something in response to
> bus locks that occurred in the guest.  E.g. if the host is taking punitive action,
> say by stalling the vCPU, then the guest kernel could bypass that behavior by
> enabling bus lock detect itself.
> 
> Maybe it's moot point in practice, since it sounds like Bus Lock Threshold will
> be available at the same time.
> 
> Ugh, and if we wanted to let the host handle guest-induced #DBs, we'd need code
> to keep Bus Lock Detect enabled in the guest since it resides in DEBUG_CTL.  Bah.
> 
> So I guess if the vcpu->guest_debug part is fairly straightforward, it probably
> makes to virtualize Bus Lock Detect because the only reason not to virtualize it
> would actually require more work/code in KVM.

KVM forwards #DB to Qemu when vcpu->guest_debug is set and it's Qemu's
responsibility to re-inject exception when Bus Lock Trap is enabled
inside the guest. I realized that it is broken so I've prepared a
Qemu patch, embedding it at the end.

> I'd still love to see Bus Lock Threshold support sooner than later though :-)

With Bus Lock Threshold enabled, I assume the changes introduced by this
patch plus Qemu fix are sufficient to support Bus Lock Trap inside the
guest?

Thanks,
Ravi

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