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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

Simply dropping the comment would be ok, but I'd also like to understand
hardware behaviour if it differs from what the APM says.

~Andrew