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

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:
> >> Now that the hypervisor intercepts all config space writes and monitors
> >> changes to the masking flags, this undoes the main effect of the
> >> XSA-129 fix, exposing the masking capability again to guests.

Could you please mention the relevant commit ids in Xen?

What happens if QEMU, with this change, is running on top of an older
Xen that doesn't intercepts all config space writes?

> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >> ---
> >> 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?

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?

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