Hi Sean,
On 24-Feb-25 11:43 PM, Sean Christopherson wrote:
> Manually zero DEBUGCTL prior to VMRUN if the host's value is non-zero and
> LBR virtualization is disabled, as hardware only context switches DEBUGCTL
> if LBR virtualization is fully enabled. Running the guest with the host's
> value has likely been mildly problematic for quite some time, e.g. it will
> result in undesirable behavior if host is running with BTF=1.
>
> But the bug became fatal with the introduction of Bus Lock Trap ("Detect"
> in kernel paralance) support for AMD (commit 408eb7417a92
> ("x86/bus_lock: Add support for AMD")), as a bus lock in the guest will
> trigger an unexpected #DB.
>
> Note, suppressing the bus lock #DB, i.e. simply resuming the guest without
> injecting a #DB, is not an option. It wouldn't address the general issue
> with DEBUGCTL, e.g. for things like BTF, and there are other guest-visible
> side effects if BusLockTrap is left enabled.
>
> If BusLockTrap is disabled, then DR6.BLD is reserved-to-1; any attempts to
> clear it by software are ignored. But if BusLockTrap is enabled, software
> can clear DR6.BLD:
>
> Software enables bus lock trap by setting DebugCtl MSR[BLCKDB] (bit 2)
> to 1. When bus lock trap is enabled, ... The processor indicates that
> this #DB was caused by a bus lock by clearing DR6[BLD] (bit 11). DR6[11]
> previously had been defined to be always 1.
>
> and clearing DR6.BLD is "sticky" in that it's not set (i.e. lowered) by
> other #DBs:
>
> All other #DB exceptions leave DR6[BLD] unmodified
>
> E.g. leaving BusLockTrap enable can confuse a legacy guest that writes '0'
> to reset DR6.
What if guest sets DEBUGCTL[BusLockTrapEn] and runs an application which
causes a bus lock? Guest will receive #DB due to bus lock, even though
guest CPUID says BusLockTrap isn't supported. Should KVM prevent guest
to write to DEBUGCTL[BusLockTrapEn]? Something like:
---
@@ -3168,6 +3168,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
if (data & DEBUGCTL_RESERVED_BITS)
return 1;
+ if ((data & DEBUGCTLMSR_BUS_LOCK_DETECT) &&
+ !guest_cpu_cap_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
+ return 1;
+
svm_get_lbr_vmcb(svm)->save.dbgctl = data;
svm_update_lbrv(vcpu);
break;
---
Thanks,
Ravi
