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Re: [Qemu-devel] [PATCH] hw/intc: fix failure return for xics_alloc_bloc


From: Greg Kurz
Subject: Re: [Qemu-devel] [PATCH] hw/intc: fix failure return for xics_alloc_block()
Date: Wed, 10 Feb 2016 10:41:16 +0100

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().

> > 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 ?

Cheers.

--
Greg

> >   
> > > ---
> > >  hw/intc/xics.c     |   10 ++++++----
> > >  hw/ppc/spapr_pci.c |    9 +++++----
> > >  trace-events       |    1 +
> > >  3 files changed, 12 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/hw/intc/xics.c b/hw/intc/xics.c
> > > index cd91ddc4d1d9..3bb77ff96e7b 100644
> > > --- a/hw/intc/xics.c
> > > +++ b/hw/intc/xics.c
> > > @@ -763,11 +763,13 @@ int xics_alloc_block(XICSState *icp, int src, int 
> > > num, bool lsi, bool align)
> > >      } else {
> > >          first = ics_find_free_block(ics, num, 1);
> > >      }
> > > +    if (first < 0) {
> > > +        trace_xics_alloc_block_failed_no_left(src, num);
> > > +        return -1;
> > > +    }
> > >  
> > > -    if (first >= 0) {
> > > -        for (i = first; i < first + num; ++i) {
> > > -            ics_set_irq_type(ics, i, lsi);
> > > -        }
> > > +    for (i = first; i < first + num; ++i) {
> > > +        ics_set_irq_type(ics, i, lsi);
> > >      }
> > >      first += ics->offset;
> > >  
> > > diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> > > index cca9257fecc5..ba33cee2a465 100644
> > > --- a/hw/ppc/spapr_pci.c
> > > +++ b/hw/ppc/spapr_pci.c
> > > @@ -275,7 +275,8 @@ static void rtas_ibm_change_msi(PowerPCCPU *cpu, 
> > > sPAPRMachineState *spapr,
> > >      unsigned int req_num = rtas_ld(args, 4); /* 0 == remove all */
> > >      unsigned int seq_num = rtas_ld(args, 5);
> > >      unsigned int ret_intr_type;
> > > -    unsigned int irq, max_irqs = 0, num = 0;
> > > +    unsigned int max_irqs = 0, num = 0;
> > > +    int irq;
> > >      sPAPRPHBState *phb = NULL;
> > >      PCIDevice *pdev = NULL;
> > >      spapr_pci_msi *msi;
> > > @@ -354,7 +355,7 @@ static void rtas_ibm_change_msi(PowerPCCPU *cpu, 
> > > sPAPRMachineState *spapr,
> > >      /* Allocate MSIs */
> > >      irq = xics_alloc_block(spapr->icp, 0, req_num, false,
> > >                             ret_intr_type == RTAS_TYPE_MSI);
> > > -    if (!irq) {
> > > +    if (irq < 0) {
> > >          error_report("Cannot allocate MSIs for device %x", config_addr);
> > >          rtas_st(rets, 0, RTAS_OUT_HW_ERROR);
> > >          return;
> > > @@ -1359,10 +1360,10 @@ static void spapr_phb_realize(DeviceState *dev, 
> > > Error **errp)
> > >  
> > >      /* Initialize the LSI table */
> > >      for (i = 0; i < PCI_NUM_PINS; i++) {
> > > -        uint32_t irq;
> > > +        int32_t irq;
> > >  
> > >          irq = xics_alloc_block(spapr->icp, 0, 1, true, false);
> > > -        if (!irq) {
> > > +        if (irq < 0) {
> > >              error_setg(errp, "spapr_allocate_lsi failed");
> > >              return;
> > >          }
> > > diff --git a/trace-events b/trace-events
> > > index c9ac144ceee4..07b0250aaf11 100644
> > > --- a/trace-events
> > > +++ b/trace-events
> > > @@ -1393,6 +1393,7 @@ xics_alloc(int src, int irq) "source#%d, irq %d"
> > >  xics_alloc_failed_hint(int src, int irq) "source#%d, irq %d is already 
> > > in use"
> > >  xics_alloc_failed_no_left(int src) "source#%d, no irq left"
> > >  xics_alloc_block(int src, int first, int num, bool lsi, int align) 
> > > "source#%d, first irq %d, %d irqs, lsi=%d, alignnum %d"
> > > +xics_alloc_block_failed_no_left(int src, int num) "source#%d, cannot 
> > > find %d consecutive irqs"
> > >  xics_ics_free(int src, int irq, int num) "Source#%d, first irq %d, %d 
> > > irqs"
> > >  xics_ics_free_warn(int src, int irq) "Source#%d, irq %d is already free"
> > >  
> > >     
> >   
> 




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