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 ---><---
