Re: [Xen-devel] [PATCH RFC v2 4/4] xen/MSI: re-expose masking capability

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.12.15 at 15:33, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> On Mon, 7 Dec 2015, Jan Beulich wrote:
>> >>> On 07.12.15 at 15:56, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> > On Mon, 7 Dec 2015, Jan Beulich wrote:
>> >> >>> On 07.12.15 at 13:45, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> >> > On Tue, 24 Nov 2015, Jan Beulich wrote:
>> >> >> TBD: We probably need to deal with running on an older hypervisor. I
>> >> >>      can't, however, immediately see a way for qemu to find out.
>> >> > 
>> >> > Actually QEMU has already an infrastructure to detect the hypervisor
>> >> > version at compile time, see include/hw/xen/xen_common.h. You could
>> >> > #define the right emu_mask depending on the hypervisor.
>> >> 
>> >> We don't want compile time detection here, but runtime one.
>> > 
>> > I guess the issue is that a fix was backported to Xen that changed its
>> > behaviour in past releases, right?
>> 
>> No, we shouldn't try to guess whether this is present in any pre-4.6
>> hypervisors; we should simply accept that maskable MSI is not
>> available for guests there, because ...
>> 
>> > Is there a way to detect the presence of this fix in Xen, by invoking an
>> > hypercall and checking the returned values and error numbers?
>> 
>> ... there's nothing to (reliably) probe here. This really is just an
>> implementation detail of the hypervisor, and hence a version check
>> is all we have available.
> 
> In that case, I think we should stay on the safe side, and only expose
> the masking capability (only take into effects the changes that this
> patch makes) for Xen >= 4.7.
> 
> What do you think?

That's what I suggested, with the hope of getting a hint where the
necessary infrastructure is (somehow I didn't find any run time
version dependent code to clone from).

Jan


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