Re: [Xen-devel] [RFC] x86/vm_event: Allow returning i-cache for emulation

[Tamas Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

On Fri, Sep 9, 2016 at 5:11 PM, Stefano Stabellini
<sstabellini@xxxxxxxxxx> wrote:
> On Fri, 9 Sep 2016, Tamas K Lengyel wrote:
>> When emulating instructions the emulator maintains a small i-cache fetched
>> from the guest memory. Under certain scenarios this memory region may contain
>> instructions that a monitor subscriber would prefer to hide, namely INT3, and
>> instead would prefer to emulate a different instruction in-place.
>>
>> This patch extends the vm_event interface to allow returning this i-cache via
>> the vm_event response.
>>
>> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
>>
>> diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
>> index 8398af7..161d149 100644
>> --- a/xen/common/vm_event.c
>> +++ b/xen/common/vm_event.c
>> @@ -407,8 +407,11 @@ void vm_event_resume(struct domain *d, struct 
>> vm_event_domain *ved)
>>              vm_event_register_write_resume(v, &rsp);
>>              break;
>>
>> +        case VM_EVENT_REASON_SOFTWARE_BREAKPOINT:
>> +            vm_event_interrupt_emulate_check(v, &rsp);
>> +            break;
>> +
>>  #ifdef CONFIG_HAS_MEM_ACCESS
>> -        case VM_EVENT_REASON_MEM_ACCESS:
>>              mem_access_resume(v, &rsp);
>>              break;
>>  #endif
>> diff --git a/xen/include/asm-arm/vm_event.h b/xen/include/asm-arm/vm_event.h
>> index ccc4b60..e56bc78 100644
>> --- a/xen/include/asm-arm/vm_event.h
>> +++ b/xen/include/asm-arm/vm_event.h
>> @@ -40,6 +40,12 @@ static inline void vm_event_toggle_singlestep(struct 
>> domain *d, struct vcpu *v)
>>  }
>>
>>  static inline
>> +void vm_event_interrupt_emulate_check(struct vcpu *v, vm_event_response_t 
>> *rsp)
>> +{
>> +    /* Not supported on ARM. */
>> +}
>> +
>> +static inline
>>  void vm_event_register_write_resume(struct vcpu *v, vm_event_response_t 
>> *rsp)
>>  {
>>      /* Not supported on ARM. */
>
> Doesn't it make sense to return some sort of error?

Not really, there is no path for that error to reach the user who
triggered this via the vm_event response. Usually if there is an error
in the operation specified by the response flag we just print a
message to the Xen console. But since these ops are not supported on
ARM at all - there is no emulator - it would be kind-of be redundant.

Tamas

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel