On Wed, Feb 26, 2025, Ravi Bangoria wrote:
> 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:
Ugh, right, AMD's legacy DEBUGCTL_RESERVED_BITS weirdness. Ideally, KVM would
make bits 5:2 reserved. I suspect we could get away with that, because VMX has
rejected all bits except BTF and LBR since the beginning. But I really, really
don't want to deal with more guest breakage due to sending such a change to
stable kernels, so for an immediate fix, I'll add a patch to drop those bits.
That'll still be a guest-visible change, e.g. if the guest is enabling LBRs *and*
the legacy PBi bits, then the state of the PBi bits would be accurate. But given
KVM's craptastic handling of DEBUGCTL, I highly doubt dropping bits 5:2 will break
anything.
*sigh*
And that's exposes yet another bug in this code. Zeroing DEBUGCTL before VMRUN
is wrong if the guest has enabled BTF. KVM should *load* the guest's desired
value if DEBUGCTL == BTF, i.e. if BTF is enabled but LBRs are not.
> ---
> @@ -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
