Re: [PATCH 2/2] x86/svm: Intercept Bus Locks for HVM guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.01.2026 14:29, Andrew Cooper wrote:
> On 20/01/2026 1:27 pm, Jan Beulich wrote:
>> On 20.01.2026 14:18, Andrew Cooper wrote:
>>> On 20/01/2026 9:53 am, Alejandro Vallejo wrote:
>>>> --- a/xen/arch/x86/hvm/svm/vmcb.c
>>>> +++ b/xen/arch/x86/hvm/svm/vmcb.c
>>>> @@ -66,6 +66,12 @@ static int construct_vmcb(struct vcpu *v)
>>>>          GENERAL2_INTERCEPT_XSETBV      | GENERAL2_INTERCEPT_ICEBP       |
>>>>          GENERAL2_INTERCEPT_RDPRU;
>>>>  
>>>> +    if ( cpu_has_bus_lock_thresh )
>>>> +    {
>>>> +        vmcb->_general3_intercepts = GENERAL3_INTERCEPT_BUS_LOCK_THRESH;
>>> |=
>>>
>>>> +        vmcb->bus_lock_thresh = 1; /* trigger immediately */
>>> Really?  The APM states:
>>>
>>> On processors that support Bus Lock Threshold (indicated by CPUID
>>> Fn8000_000A_EDX[29] BusLockThreshold=1), the VMCB provides a Bus Lock
>>> Threshold enable bit and an unsigned 16-bit Bus Lock Threshold count. On
>>> VMRUN, this value is loaded into an internal count register. Before the
>>> processor executes a bus lock in the guest, it checks the value of this
>>> register. If the value is greater than 0, the processor executes the bus
>>> lock successfully and decrements the count. If the value is 0, the bus
>>> lock is not executed and a #VMEXIT to the VMM is taken.
>>>
>>> So according to the APM, setting the count to 1 will permit one bus lock
>>> then exit (fault style) immediately before the next.  This also says
>>> that a count of 0 is a legal state.
>> But then you'd livelock the guest as soon as it uses a bus lock. Are you
>> suggesting to set to 1 in response to a bus lock exit, and keep at 0 at
>> all other times?
> 
> I should have been clearer.  I'm complaining at the "trigger
> immediately" comment, because I don't think that's a correct statement
> of how hardware behaves.

In turn I should have looked at the patch itself before commenting. The
other setting to 1 is what makes sense, and what ought to prevent a
livelock. The one here indeed raises questions.

Jan