Re: [Qemu-devel] [PATCH for-4.2] xics/kvm: Convert assert() to error_setg()

[Greg Kurz <groug@kaod.org>]

On Fri, 5 Jul 2019 14:56:23 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Jul 04, 2019 at 10:12:04AM +0200, Greg Kurz wrote:
> > On Thu, 4 Jul 2019 10:23:57 +1000
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> > 
> > > On Wed, Jul 03, 2019 at 07:50:12PM +0200, Greg Kurz wrote:
> > > > ics_set_kvm_state_one() is called either during reset, in which case
> > > > both 'saved priority' and 'current priority' are equal to 0xff, or
> > > > during migration. In the latter case, 'saved priority' may differ
> > > > from 'current priority' only if the interrupt had been masked with
> > > > the ibm,int-off RTAS call. Instead of aborting QEMU, print out an
> > > > error and exit.
> > > 
> > > What's the rationale for this?  Doesn't hitting this indicate an error
> > > in the qemu code, for which an abort is the usual response?
> > > 
> > 
> > This error can be hit by the destination during migration if the
> > incoming stream is corrupted. Aborting in this case would mislead
> > the user into suspecting a bug in the destination QEMU, which isn't
> > the case.
> 
> Rather than a bug in the source qemu?  I guess so.
> 

A bug in the source QEMU for live migration or a corrupted snapshot
for load_vm, which could result from a qcow2 file corruption for
example.

> > Appart from that, when the in-kernel XICS is in use, only two functions
> > manipulate the ICS state: ics_set_kvm_state_one() and ics_get_kvm_state().
> > The code is trivial enough that I don't see a great value in the assert
> > in the first place... BTW, it comes from the commit:
> > 
> > commit 11ad93f68195f68cc94d988f2aa50b4d190ee52a
> > Author: David Gibson <david@gibson.dropbear.id.au>
> > Date:   Thu Sep 26 16:18:44 2013 +1000
> > 
> >     xics-kvm: Support for in-kernel XICS interrupt controller
> > 
> > Maybe you remember some context that justified the assert at the
> > time ?
> 
> It was probably mostly about documenting the invariants that are
> supposed to apply here.
> 

Indeed this error on the reset path is very likely a bug in QEMU,
and the assert() makes sense in this case.

I'm convinced by the documenting argument. Please forget this patch :)

> > 
> > > > 
> > > > Based-on: <156217454083.559957.7359208229523652842.stgit@bahia.lan>
> > > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > > ---
> > > > 
> > > > This isn't a bugfix, hence targetting 4.2, but it depends on an actual
> > > > fix for 4.1, as mentionned in the Based-on tag.
> > > > ---
> > > >  hw/intc/xics_kvm.c |   17 +++++++++++++++--
> > > >  1 file changed, 15 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/hw/intc/xics_kvm.c b/hw/intc/xics_kvm.c
> > > > index 2df1f3e92c7e..f8758b928250 100644
> > > > --- a/hw/intc/xics_kvm.c
> > > > +++ b/hw/intc/xics_kvm.c
> > > > @@ -255,8 +255,21 @@ int ics_set_kvm_state_one(ICSState *ics, int srcno, Error **errp)
> > > >      state = irq->server;
> > > >      state |= (uint64_t)(irq->saved_priority & KVM_XICS_PRIORITY_MASK)
> > > >          << KVM_XICS_PRIORITY_SHIFT;
> > > > -    if (irq->priority != irq->saved_priority) {
> > > > -        assert(irq->priority == 0xff);
> > > > +
> > > > +    /*
> > > > +     * An interrupt can be masked either because the ICS is resetting, in
> > > > +     * which case we expect 'current priority' and 'saved priority' to be
> > > > +     * equal to 0xff, or because the guest has called the ibm,int-off RTAS
> > > > +     * call, in which case we we have recorded the priority the interrupt
> > > > +     * had before it was masked in 'saved priority'. If the interrupt isn't
> > > > +     * masked, 'saved priority' and 'current priority' are equal (see
> > > > +     * ics_get_kvm_state()). Make sure we restore a sane state, otherwise
> > > > +     * fail migration.
> > > > +     */
> > > > +    if (irq->priority != irq->saved_priority && irq->priority != 0xff) {
> > > > +        error_setg(errp, "Corrupted state detected for interrupt source %d",
> > > > +                   srcno);
> > > > +        return -EINVAL;
> > > >      }
> > > >  
> > > >      if (irq->priority == 0xff) {
> > > > 
> > > 
> > 
>