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@xxxxxxx> 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@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? >> >> 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@xxxxxxxxxx Sure, I'll add a patch and respin the series. Thanks, Ravi
