linux-kernel - Re: [PATCH v2 2/2] vfio/pci: Don't regenerate vconfig for all BARs if !bardirty

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Message-ID: <20200921122134.5c7794f3.cohuck@redhat.com>
Date:   Mon, 21 Sep 2020 12:21:34 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Zenghui Yu <yuzenghui@...wei.com>
Cc:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <alex.williamson@...hat.com>, <wanghaibin.wang@...wei.com>
Subject: Re: [PATCH v2 2/2] vfio/pci: Don't regenerate vconfig for all BARs
 if !bardirty

On Mon, 21 Sep 2020 12:51:16 +0800
Zenghui Yu <yuzenghui@...wei.com> wrote:

> Now we regenerate vconfig for all the BARs via vfio_bar_fixup(), every time
> any offset of any of them are read. Though BARs aren't re-read regularly,
> the regeneration can be avoid if no BARs had been written since they were

s/avoid/avoided/

> last read, in which case the vdev->bardirty is false.

s/the//

> 
> Let's predicate the vfio_bar_fixup() on the bardirty so that it can return
> immediately if !bardirty.

Maybe

"Let's return immediately in vfio_bar_fixup() if bardirty is false." ?

> 
> Suggested-by: Alex Williamson <alex.williamson@...hat.com>
> Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>
> ---
> * From v1:
>   - Per Alex's suggestion, let vfio_bar_fixup() test vdev->bardirty to
>     avoid doing work if bardirty is false, instead of removing it entirely.
>   - Rewrite the commit message.
> 
>  drivers/vfio/pci/vfio_pci_config.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> index d98843feddce..5e02ba07e8e8 100644
> --- a/drivers/vfio/pci/vfio_pci_config.c
> +++ b/drivers/vfio/pci/vfio_pci_config.c
> @@ -467,6 +467,9 @@ static void vfio_bar_fixup(struct vfio_pci_device *vdev)
>  	__le32 *vbar;
>  	u64 mask;
> 
> +	if (!vdev->bardirty)

Finally, bardirty can actually affect something :)

> +		return;
> +
>  	vbar = (__le32 *)&vdev->vconfig[PCI_BASE_ADDRESS_0];
>  
>  	for (i = 0; i < PCI_STD_NUM_BARS; i++, vbar++) {

Reviewed-by: Cornelia Huck <cohuck@...hat.com>