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
