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

[Zenghui Yu]

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);


Thanks,
Zenghui