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

Ravi Bangoria
2024-07-11T08:52:06Z

On 10-Jul-24 7:22 PM, Sean Christopherson wrote:
> On Tue, Jul 09, 2024, Jim Mattson wrote:
>> On Tue, Jul 9, 2024 at 7:25 PM Ravi Bangoria <ravi.bangoria@amd.com> wrote:
>>>
>>> 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@amd.com
>>>
>>> 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?
>>
>> In any case, it seems that commit 76ea438b4afc ("KVM: X86: Expose bus
>> lock debug exception to guest") prematurely advertised the presence of
>> X86_FEATURE_BUS_LOCK to userspace on non-Intel platforms. We should
>> probably either accept these changes or fix up that commit. Either
>> way, something should be done for all active branches back to v5.15.
>
> Drat. Yeah, we need a patch to clear BUS_LOCK_DETECT in svm_set_cpu_caps(), marked
> for stable@. Then this series can remove that clearing.
>
> At least I caught it for CET[*]! It'd be nice to not have to rely on humans to
> detect potential issues like this, but I can't think of a way to programmatically
> handle this situation without incurring an annoying amount of overhead and/or
> duplicate code between VMX and SVM.
>
> [*] https://lore.kernel.org/all/ZjLRnisdUgeYgg8i@google.com

Sure, I'll add a patch and respin the series.

Thanks,
Ravi