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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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:
> >> >> 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?
> 
> It's just one (which I've now  added a reference to), unless you want
> all the prereqs listed.

One is probably OK.


> > What happens if QEMU, with this change, is running on top of an older
> > Xen that doesn't intercepts all config space writes?
> 
> The security issue would resurface.
> 
> >> >> 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?
> 
> 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?

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