Messages in this thread | | | Subject | Re: [PATCH 2/2] vfio/pci: Remove bardirty from vfio_pci_device | From | Zenghui Yu <> | Date | Sat, 19 Sep 2020 10:39:55 +0800 |
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On 2020/9/19 10:11, Alex Williamson wrote: > On Sat, 19 Sep 2020 09:54:00 +0800 > Zenghui Yu <yuzenghui@huawei.com> wrote: > >> 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); > > > 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
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