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