Messages in this thread Patch in this message | | | Subject | Re: [PATCH 2/2] vfio/pci: Remove bardirty from vfio_pci_device | From | Zenghui Yu <> | Date | Sat, 19 Sep 2020 09:54:00 +0800 |
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Hi Alex,
On 2020/9/18 6:07, Alex Williamson wrote: > On Thu, 17 Sep 2020 13:35:37 +0200 > Cornelia Huck <cohuck@redhat.com> wrote: > >> On Thu, 17 Sep 2020 11:31:28 +0800 >> Zenghui Yu <yuzenghui@huawei.com> 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@huawei.com> >>> --- >>> 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
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