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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 11 Dec 2015, Jan Beulich wrote:
> >>> 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).
 
It is not possible to do this at runtime. I think we should do this at
compile time because in any case it is not supported to run a QEMU built
for a given Xen version on a different Xen version.

The infrastructure to do this at compile time is in
./include/hw/xen/xen_common.h

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