Re: [Xen-devel] xc_evtchn_status fails with EFAULT on HVM, the same on PV works

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Jan 13, 2017 at 05:38:42PM +0000, Andrew Cooper wrote:
> On 13/01/17 17:31, Marek Marczykowski-Górecki wrote:
> > Hi,
> >
> > I have a strange problem - xc_evtchn_status fails when running in HVM,
> > while exactly the same code (same kernel, same application etc) works
> > fine in PV. I've narrowed it down to copy_from_guest call in
> > common/event_channel.c, but no idea why it fails there. Xen version is
> > 4.8.0. kernel is kernel-4.8.13-100.fc23. Any idea?
> 
> Which specific copy_from_guest() call?
> 
> Copying data out of a PV guest is different to copying out of a HVM
> guest, but copy_from_guest() should cope properly with both.
> 
> However, to progress, it would help to know exactly which piece of data
> is being requested.

This one:
https://github.com/xen-project/xen/blob/stable-4.8/xen/common/event_channel.c#L1104

    case EVTCHNOP_status: {
        struct evtchn_status status;
        if ( copy_from_guest(&status, arg, 1) != 0 )
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            return -EFAULT;
        rc = evtchn_status(&status);
        if ( !rc && __copy_to_guest(arg, &status, 1) )
            rc = -EFAULT;
        break;

The evtchn_status structure in application is on stack (local variable),
but I think it shouldn't matter, as libxc copy it to a bounce buffer.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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