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Re: [PATCH 2/2] x86/svm: Intercept Bus Locks for HVM guests
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 20 Jan 2026 14:29:14 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 20 Jan 2026 13:29:21 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 20.01.2026 14:11, Teddy Astie wrote:
> Le 20/01/2026 à 10:56, Alejandro Vallejo a écrit :
>> --- 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 */
>> + }
>> +
>> /* Intercept all debug-register writes. */
>> vmcb->_dr_intercepts = ~0u;
>>
>
> According to APM,
>
> INTERCEPT_BUS_LOCK_THRESH does
> > Intercept bus lock operations when Bus Lock Threshold Counter is 0
>
> I assume that when set to 0, we intercept all bus locks, so if set to 1,
> every 2 bus lock (since we first go from 1 to 0, then at 0 we intercept
> the next one) ?
>
> I think we want that to be tunable, as intercepting all bus locks may be
> too extreme we probably want to intercept every few ones instead.
Otoh bus locks (as opposed to cache locks) would better be rare, or else
perhaps such a guest deserves some extra slowing down?
Jan
|
