Re: [Xen-devel] [PATCH] x86/monitor: Disallow setting mem_access_emulate_each_rep when vm_event is NULL

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/04/2016 08:16, Razvan Cojocaru wrote:
> On 04/08/16 10:10, Razvan Cojocaru wrote:
>> It is meaningless (and potentially dangerous - see 
>> hvmemul_virtual_to_linear())
>> to set mem_access_emulate_each_rep before xc_monitor_enable() (which 
>> allocates
>> vcpu->arch.vm_event) has been called, so return an error from the
>> XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP hypercall when that is the case.
>>
>> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>> ---
>>  xen/include/asm-x86/monitor.h | 15 +++++++++++++--
>>  1 file changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/xen/include/asm-x86/monitor.h b/xen/include/asm-x86/monitor.h
>> index 0954b59..0544836 100644
>> --- a/xen/include/asm-x86/monitor.h
>> +++ b/xen/include/asm-x86/monitor.h
>> @@ -35,11 +35,22 @@ int arch_monitor_domctl_op(struct domain *d, struct 
>> xen_domctl_monitor_op *mop)
>>      switch ( mop->op )
>>      {
>>      case XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP:
>> +    {
>> +        bool_t value = !!mop->event;

While you are at at, one extra newline here please.

>>          domain_pause(d);
>> -        d->arch.mem_access_emulate_each_rep = !!mop->event;
>> +        /*
>> +         * Enabling emulate_each_rep without a vm_event subscriber
>> +         * is meaningless.
>> +         */
>> +        if ( !d->vcpu || !d->vcpu[0]->arch.vm_event )
> Sorry, this is a bit convoluted, and wrong: if d->vcpu != NULL it will
> return -EINVAL even if d->vcpu[0]->arch.vm_event is NULL. I'll rework
> it. Apologies for the noise.

The issue comes about because d->arch.mem_access_emulate_each_rep is a
per-domain variable, whereas vm_event listeners are per-vcpu.

In principle, it would be legitimate for an introspection engine to only
want to introspect a single vcpu.

Either we need to enforce that every vcpu has vm_event infrastructure
set up (and allow for delivery of events is disabled on a per-vcpu
basis), or settings such as emulate_each_rep needs to be made per-vcpu.

~Andrew

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