Re: [PATCH 2/3] KVM: SVM: Manually zero/restore DEBUGCTL if LBR virtualization is disabled

[Sean Christopherson <seanjc@google.com>]

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