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

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Message-ID: <20200918201128.16cf0a1c@x1.home>
Date:   Fri, 18 Sep 2020 20:11:28 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Zenghui Yu <yuzenghui@...wei.com>
Cc:     Cornelia Huck <cohuck@...hat.com>, <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

On Sat, 19 Sep 2020 09:54:00 +0800
Zenghui Yu <yuzenghui@...wei.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@...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);


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,

Alex