On Wed, Jun 05, 2024, Ravi Bangoria wrote: > Hi Sean, > > On 6/4/2024 6:15 AM, Sean Christopherson wrote: > > On Mon, Apr 29, 2024, Ravi Bangoria wrote: > >> Upcoming AMD uarch will support Bus Lock Detect. Add support for it > >> in KVM. Bus Lock Detect is enabled through MSR_IA32_DEBUGCTLMSR and > >> MSR_IA32_DEBUGCTLMSR is virtualized only if LBR Virtualization is > >> enabled. Add this dependency in the KVM. > > > > This is woefully incomplete, e.g. db_interception() needs to be updated to decipher > > whether the #DB is the responsbility of the host or of the guest. > > Can you please elaborate. Are you referring to vcpu->guest_debug thingy? Yes. More broadly, all of db_interception(). > > Honestly, I don't see any point in virtualizing this in KVM. As Jim alluded to, > > what's far, far more interesting for KVM is "Bus Lock Threshold". Virtualizing > > this for the guest would have been nice to have during the initial split-lock #AC > > support, but now I'm skeptical the complexity is worth the payoff. > > This has a valid usecase of penalizing offending processes. I'm not sure > how much it's really used in the production though. Yeah, but split-lock #AC and #DB have existed on Intel for years, and no one has put in the effort to land KVM support, despite the series getting as far as v9[*]. 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. Anyways, my point is that, except for SEV-ES+ where there's no good reason NOT to virtualize Bus Lock Detect, I'm not convinced that it's worth virtualizing bus-lock #DBs. [*] https://lore.kernel.org/all/20200509110542.8159-1-xiaoyao.li@xxxxxxxxx > > I suppose we could allow it if #DB isn't interecepted, at which point the enabling > > required is minimal? > > The feature uses DEBUG_CTL MSR, #DB and DR6 register. Do you mean expose > it when all three are accelerated or just #DB? I mean that if KVM isn't intercepting #DB, then there's no extra complexity needed to sort out whether the #DB "belongs" to the host or the guest. See commit 90cbf6d914ad ("KVM: SEV-ES: Eliminate #DB intercept when DebugSwap enabled").
