Re: [Xen-devel] [PATCH v2] x86/vm_event: toggle singlestep from vm_event response

["Lengyel, Tamas" <tlengyel@xxxxxxxxxxx>]

On Mon, Jul 6, 2015 at 1:29 PM, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> wrote:

On 07/06/2015 08:17 PM, Andrew Cooper wrote:
> On 06/07/15 18:08, Lengyel, Tamas wrote:
>>
>>     Having said that (and with the understading that it is beyond the
>>     scope
>>     of this patch), a way to validate things like these is a good idea. I
>>     wonder if, in a future patch, we could not have ./configure detect
>>     these
>>     things and simply disable the relevant VM_EVENT_FLAG constants with
>>     #if(n)defs, for example. That way, you wouldn't be able to compile
>>     code
>>     that wouldn't work silently on platforms where that is the case.
>>
>>
>> It would be something worth investigating, definitely.
>
> It would be mad to conditionally compile out code based on the features
> or lackthereof of the build machine.
>
> For bits like this, there must be active negotiation between userspace
> and the running hypervisor to see what it can support.  Imagine if the
> user disabled the monitor trap feature in the BIOS?  Userspace cannot
> possibly assume that because it is running on Intel, that the feature is
> present and usable.

Fair enough, but it would at least compile out the code for machines
where it _definitely_ won't work, and on those where it _might_ work it
would just continue to do nothing silently (or perhaps with the
hypervisor logging an error) once the user disables the monitor trap
flag in the BIOS, so while not perfect it's still something, with the
benefit of less development overhead than a full-on system for
negotiating hypervisor capabilities (which is indeed the safest and
exhaustive course of action).

Just a thought,
Razvan

I could see a new XEN_DOMCTL_MONITOR_OP_GET_CAPABILITIES making sense for this so userspace would get a definite yes/no for features available on the given platform. If the user then decides to ignore and attempt to use something not available, that would be on him.

Tamas

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