Re: [PATCH 2/2] vfio/pci: Remove bardirty from vfio_pci_device

[Zenghui Yu]

On 2020/9/19 10:11, Alex Williamson wrote:
> On Sat, 19 Sep 2020 09:54:00 +0800
> Zenghui Yu <yuzenghui@xxxxxxxxxx> wrote:
>
> > Hi Alex,
> >
> > On 2020/9/18 6:07, Alex Williamson wrote:
> > > On Thu, 17 Sep 2020 13:35:37 +0200
> > > Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
> > >    
> > > > On Thu, 17 Sep 2020 11:31:28 +0800
> > > > Zenghui Yu <yuzenghui@xxxxxxxxxx> wrote:
> > > >   
> > > > > It isn't clear what purpose the @bardirty serves. It might be used to avoid
> > > > > the unnecessary vfio_bar_fixup() invoking on a user-space BAR read, which
> > > > > is not required when bardirty is unset.
> > > > >
> > > > > The variable was introduced in commit 89e1f7d4c66d ("vfio: Add PCI device
> > > > > driver") but never actually used, so it shouldn't be that important. Remove
> > > > > it.
> > > > >
> > > > > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > > > > ---
> > > > >    drivers/vfio/pci/vfio_pci_config.c  | 7 -------
> > > > >    drivers/vfio/pci/vfio_pci_private.h | 1 -
> > > > >    2 files changed, 8 deletions(-)
> > > >
> > > > Yes, it seems to have been write-only all the time.
> > >
> > > I suspect the intent was that vfio_bar_fixup() could test
> > > vdev->bardirty to avoid doing work if no BARs had been written since
> > > they were last read.  As it is now we regenerate vconfig for all the
> > > BARs every time any offset of any of them are read.  BARs aren't
> > > re-read regularly and config space is not a performance path,
> >
> > Yes, it seems that Qemu itself emulates all BAR registers and will read
> > the BAR from the kernel side only at initialization time.
> >
> > > but maybe
> > > we should instead test if we see any regressions from returning without
> > > doing any work in vfio_bar_fixup() if vdev->bardirty is false.  Thanks,
> >
> > I will test it with the following diff. Please let me know which way do
> > you prefer.
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_config.c
> > b/drivers/vfio/pci/vfio_pci_config.c
> > index d98843feddce..77c419d536d0 100644
> > --- a/drivers/vfio/pci/vfio_pci_config.c
> > +++ b/drivers/vfio/pci/vfio_pci_config.c
> > @@ -515,7 +515,7 @@ static int vfio_basic_config_read(struct
> > vfio_pci_device *vdev, int pos,
> >                                     int count, struct perm_bits *perm,
> >                                     int offset, __le32 *val)
> >    {
> > -       if (is_bar(offset)) /* pos == offset for basic config */
> > +       if (is_bar(offset) && vdev->bardirty) /* pos == offset for basic
> > config */
> >                   vfio_bar_fixup(vdev);
> >
> >           count = vfio_default_config_read(vdev, pos, count, perm,
> > offset, val);
>
>
> There's only one caller currently, but I'd think it cleaner to put this
> in vfio_bar_fixup(), ie. return immediately if !bardirty.  Thanks,

OK, I'll do that in the v2.


Thanks,
Zenghui