linux-kernel - Re: [PATCH] PCI/IOV: Decrease VF memory BAR size to save host memory occupied by PTEs

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Message-ID: <Y0VVXB0XTobClVJo@unreal>
Date:   Tue, 11 Oct 2022 14:37:00 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Rui Ma <Rui.Ma@....com>
Cc:     helgaas@...nel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, Alexander.Deucher@....com,
        bhelgaas@...gle.com
Subject: Re: [PATCH] PCI/IOV: Decrease VF memory BAR size to save host memory
 occupied by PTEs

On Tue, Oct 11, 2022 at 07:23:25PM +0800, Rui Ma wrote:
> In some certain SR-IOV scene, when the device physical space(such as Video
> RAM)is fixed, as the number of VFs increases, some device driver may decrease
> actual BAR memory space used by each VF. However, the VF BAR memory mapping is
> always based on the usual BAR probing algorithm in PCIe spec. So do not map this
> unneeded memory can save host memory which occupied by PTEs. Although each PTE
> only occupies a few bytes of space on its own, a large number of PTEs can still
> take up a lot of space.
> 
> Signed-off-by: Rui Ma <Rui.Ma@....com>
> ---
>  drivers/pci/iov.c    | 14 ++++++++++++--
>  drivers/pci/pci.h    | 15 +++++++++++++++
>  drivers/pci/quirks.c | 37 +++++++++++++++++++++++++++++++++++++
>  3 files changed, 64 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> index 952217572113..92a69e51d85c 100644
> --- a/drivers/pci/iov.c
> +++ b/drivers/pci/iov.c
> @@ -296,6 +296,14 @@ int pci_iov_add_virtfn(struct pci_dev *dev, int id)
>  	struct pci_sriov *iov = dev->sriov;
>  	struct pci_bus *bus;
>  
> +    /*
> +     * Some SR-IOV device's BAR map range is larger than they can actually use.
> +     * This extra BAR space occupy too much reverse mapping size(physical page
> +     * back to the PTEs). So add a divisor shift parameter to resize the request
> +     * resource of VF according to num of VFs.
> +     */
> +	u16 shift = 1;
> +
>  	bus = virtfn_add_bus(dev->bus, pci_iov_virtfn_bus(dev, id));
>  	if (!bus)
>  		goto failed;
> @@ -328,8 +336,10 @@ int pci_iov_add_virtfn(struct pci_dev *dev, int id)
>  		virtfn->resource[i].name = pci_name(virtfn);
>  		virtfn->resource[i].flags = res->flags;
>  		size = pci_iov_resource_size(dev, i + PCI_IOV_RESOURCES);
> +		shift = 1;
> +		shift = virtfn_get_shift(dev, iov->num_VFs, i);
>  		virtfn->resource[i].start = res->start + size * id;
> -		virtfn->resource[i].end = virtfn->resource[i].start + size - 1;
> +		virtfn->resource[i].end = virtfn->resource[i].start + (size >> (shift - 1)) - 1;
>  		rc = request_resource(res, &virtfn->resource[i]);
>  		BUG_ON(rc);
>  	}
> @@ -680,12 +690,12 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
>  	msleep(100);
>  	pci_cfg_access_unlock(dev);
>  
> +	iov->num_VFs = nr_virtfn;
>  	rc = sriov_add_vfs(dev, initial);
>  	if (rc)
>  		goto err_pcibios;
>  
>  	kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
> -	iov->num_VFs = nr_virtfn;
>  
>  	return 0;
>  
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 3d60cabde1a1..befc67a280eb 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -603,6 +603,21 @@ static inline int pci_dev_specific_reset(struct pci_dev *dev, bool probe)
>  }
>  #endif
>  
> +struct virtfn_get_shift_methods {
> +	u16 vendor;
> +	u16 device;
> +	u16 (*get_shift)(struct pci_dev *dev, u16 arg, int arg2);
> +};
> +
> +#ifdef CONFIG_PCI_QUIRKS
> +u16 virtfn_get_shift(struct pci_dev *dev, u16 arg1, int arg2);
> +#else
> +static inline u16 virtfn_get_shift(struct pci_dev *dev, u16 arg1, int arg2)
> +{
> +	return (u16)1;

<...>

> +	return (u16)1;

Why do you need these casts? You can omit them.

Thanks

> +}
> +
>  static void quirk_dma_func0_alias(struct pci_dev *dev)
>  {
>  	if (PCI_FUNC(dev->devfn) != 0)
> -- 
> 2.25.1
>