linux-kernel - Re: [PATCH 2/2] vfio/pci: Remove bardirty from vfio_pci

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Message-ID: <3b5214f9-9e17-2bcd-1b92-57bacc1c1b31@huawei.com>
Date:   Sat, 19 Sep 2020 09:54:00 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <wanghaibin.wang@...wei.com>
Subject: Re: [PATCH 2/2] vfio/pci: Remove bardirty from vfio_pci_device

Hi Alex,

On 2020/9/18 6:07, Alex Williamson wrote:
> On Thu, 17 Sep 2020 13:35:37 +0200
> Cornelia Huck <cohuck@...hat.com> wrote:
> 
>> On Thu, 17 Sep 2020 11:31:28 +0800
>> Zenghui Yu <yuzenghui@...wei.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@...wei.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