Re: [Qemu-devel] [Qemu-ppc] [PATCH] hw/intc: fix failure return for xics_alloc_block()

[Greg Kurz]

On Wed, 24 Feb 2016 11:36:49 +1100
David Gibson <address@hidden> wrote:

> On Tue, Feb 23, 2016 at 06:46:44PM +0100, Greg Kurz wrote:
> > On Wed, 10 Feb 2016 10:41:16 +0100
> > Greg Kurz <address@hidden> wrote:
> >   
> > > On Mon, 8 Feb 2016 09:31:49 +0100
> > > Greg Kurz <address@hidden> wrote:
> > >   
> > > > On Mon, 8 Feb 2016 11:45:19 +1000
> > > > David Gibson <address@hidden> wrote:
> > > >     
> > > > > On Fri, Feb 05, 2016 at 09:43:40AM +0100, Greg Kurz wrote:      
> > > > > > From: Brian W. Hart <address@hidden>
> > > > > > 
> > > > > > xics_alloc_block() does not return a clear error code when it
> > > > > > fails to allocate a block of interrupts. Instead it returns the
> > > > > > base interrupt number minus 1. This change updates it to return a
> > > > > > clear -1 in case of failure (following the example of xics_alloc()).
> > > > > > 
> > > > > > The two callers of xics_alloc_block() are updated to check for
> > > > > > a negative return as an error. They had previously checked for
> > > > > > a 0 return as an error, which wrongly treated most failures as
> > > > > > successes.
> > > > > > 
> > > > > > Fixes: bee763dbfb8cfceea112131970da07f215f293a6
> > > > > > Signed-off-by: Brian W. Hart <address@hidden>
> > > > > > [only pass src and num to trace_xics_alloc_block_failed_no_left,
> > > > > >  added trace_xics_alloc_block_failed_no_left definition to 
> > > > > > trace-events]
> > > > > > Signed-off-by: Greg Kurz <address@hidden>        
> > > > > 
> > > > > Hrm, it would probably be better to give xics_alloc_block() an Error
> > > > > ** argument so it can report errors using the new API.
> > > > >       
> > > > 
> > > > Sure. I can rework the patch to do so.
> > > >     
> > > 
> > > The trace_xics_alloc_block_failed_no_left trace is more a debugging thing
> > > than an error to be reported to the user. Also, rtas_ibm_change_msi()
> > > already has a meaningful error message:
> > > 
> > >         error_report("Cannot allocate MSIs for device %x", config_addr);
> > > 
> > > So in the end, I'm not sure about the benefit of passing an Error **
> > > down to xics_alloc_block().
> > >   
> > 
> > Hi David !
> > 
> > Given the remarks above, do you still think we should pass Error ** ?  
> 
> I still think using the Error API would be preferable, but it doesn't
> make a huge difference.
> 
> > > > > TBH the whole xics_alloc_block() interface is kind of dubious, or at
> > > > > least the ics_find_free_block() part of it.  Dynamically allocating
> > > > > irqs to devices is basically awful for migration, so it's better to
> > > > > have fixed allocations of all interrupts at the machine level.
> > > > >       
> > > > 
> > > > I agree about the extra complexity, but isn't it the purpose of
> > > > the ibm,change-msi RTAS call ? I'm not sure to understand what you
> > > > are suggesting...
> > > >     
> > > 
> > > And anyway, even if the decision is made one day to have fixed
> > > allocations, shouldn't we consider fixing this bug first ?
> > >   
> > 
> > According to the following commit changelog, the dynamic allocation was
> > introduced to support PCI hot(un)plug. Maybe Alexey may explain why it
> > got coded this way.
> > 
> > commit bee763dbfb8cfceea112131970da07f215f293a6
> > Author: Alexey Kardashevskiy <address@hidden>
> > Date:   Fri May 30 19:34:15 2014 +1000
> > 
> >     spapr: Move interrupt allocator to xics
> > 
> > I'm not sure of the amount of reflexion and work needed to address your
> > concern... Given the time frame, what about deferring xics rework to 2.7
> > and fix the bug we currently have in 2.6 ?  
> 
> Yeah, sorry, I realised after I sent that the allocation does
> actually make sense in this case - these are guest triggered
> allocations that we really do need an allocator for, not host setup
> allocations which we have used in the past but turned out to be
> dubious.
> 

There are other issues related to xics actually. I'll post a series
with all the fixes.

Cheers.

--
Greg