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Re: [PATCH 2/2] x86/svm: Intercept Bus Locks for HVM guests
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 20 Jan 2026 14:27:01 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>, Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Tue, 20 Jan 2026 13:27:11 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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?
Jan
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