[Qemu-devel] Re: [PATCH 1/8] pci: pci_default_cap_write_config ignores wmask

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 11, 2010 at 11:03:19PM -0700, Alex Williamson wrote:
> On Fri, 2010-11-12 at 07:22 +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 11, 2010 at 07:55:01PM -0700, Alex Williamson wrote:
> > > Make use of wmask, just like the rest of config space.
> > > 
> > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > ---
> > > 
> > >  hw/pci.c |   19 ++++++++-----------
> > >  1 files changed, 8 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/hw/pci.c b/hw/pci.c
> > > index 92aaa85..12c47ac 100644
> > > --- a/hw/pci.c
> > > +++ b/hw/pci.c
> > > @@ -1175,13 +1175,14 @@ uint32_t pci_default_read_config(PCIDevice *d,
> > >      return pci_read_config(d, address, len);
> > >  }
> > >  
> > > -static void pci_write_config(PCIDevice *pci_dev,
> > > -                             uint32_t address, uint32_t val, int len)
> > > +static void pci_write_config(PCIDevice *d, uint32_t addr, uint32_t val, int l)
> > >  {
> > >      int i;
> > > -    for (i = 0; i < len; i++) {
> > > -        pci_dev->config[address + i] = val & 0xff;
> > > -        val >>= 8;
> > > +    uint32_t config_size = pci_config_size(d);
> > > +
> > > +    for (i = 0; i < l && addr + i < config_size; val >>= 8, ++i) {
> > > +        uint8_t wmask = d->wmask[addr + i];
> > > +        d->config[addr + i] = (d->config[addr + i] & ~wmask) | (val & wmask);
> > >      }
> > >  }
> > >
> > 
> > 
> > Let's not name an internal static helper pci_write_config.
> > This is really update_config_by_mask or something like that.
> > But see below: maybe we don't need it at all?
> 
> The function already exists, I just made it do what it seems like it
> should have been doing already.

Yep. But since you are rewriting this function - could you rename it as
well?

> > > @@ -1207,18 +1208,14 @@ void pci_default_cap_write_config(PCIDevice *pci_dev,
> > >  
> > >  void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val, int l)
> > >  {
> > > -    int i, was_irq_disabled = pci_irq_disabled(d);
> > > -    uint32_t config_size = pci_config_size(d);
> > > +    int was_irq_disabled = pci_irq_disabled(d);
> > >  
> > >      if (pci_access_cap_config(d, addr, l)) {
> > >          d->cap.config_write(d, addr, val, l);
> > >          return;
> > >      }
> > >  
> > 
> > I would like to also examine the need for _cap_
> > functions. Why can assigned devices just do
> > 
> > 	pci_default_write_config 
> > 	if (range_overlap(...msi)) {
> > 	}
> > 	if (range_overlap(...msix)) {
> > 	}
> > and then we could remove all the _cap_ extensions
> > altogether?
> 
> I think that somewhere we need to track what capabilities are at what
> offset, config space isn't a performance path, but that look horribly
> inefficient and gets worse with more capabilities.

Looks like premature optimization to me.  I guess when we get more than
say 8 capabilities to support, I'll start to worry.
Even then, these optimizations are better internal in pci core.

> Why don't we define capability id 0xff as normal config space (first 64
> bytes), then add the capability id to read/write_config (this is what
> vfio does).  Then the driver can split capability handling off from
> their main functions if they want.

My feeling is we need higher level APIs than 'capability write'.
Otherwise we get the PCI config handling all over the place.
E.g. a callback when msi is enabled/disabled would make sense,
so that pci core can keep track of current state and only notify
the device when there are things to do.

>  Anyway, I think such an improvement
> could be added incrementally later.  Thanks,
> 
> Alex

Sure.

> > > -    for (i = 0; i < l && addr + i < config_size; val >>= 8, ++i) {
> > > -        uint8_t wmask = d->wmask[addr + i];
> > > -        d->config[addr + i] = (d->config[addr + i] & ~wmask) | (val & wmask);
> > > -    }
> > > +    pci_write_config(d, addr, val, l);
> > >  
> > >  #ifdef CONFIG_KVM_DEVICE_ASSIGNMENT
> > >      if (kvm_enabled() && kvm_irqchip_in_kernel() &&
> 
>