linux-kernel - Re: [PATCH 2/2] PCI/IOV: Assume SR-IOV VFs support extended config space.

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Date:   Wed, 19 Jun 2019 12:54:55 -0700
From:   sathyanarayanan kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        linux-pci@...r.kernel.org
Cc:     KarimAllah Ahmed <karahmed@...zon.de>,
        Hao Zheng <yinhe@...ux.alibaba.com>, bhelgaas@...gle.com,
        linux-kernel@...r.kernel.org, nanhai.zou@...ux.alibaba.com,
        quan.xu0@...ux.alibaba.com, ashok.raj@...el.com,
        keith.busch@...el.com, mike.campin@...el.com
Subject: Re: [PATCH 2/2] PCI/IOV: Assume SR-IOV VFs support extended config
 space.

Hi,

On 6/13/19 3:57 PM, Alex Williamson wrote:
> The SR-IOV specification requires both PFs and VFs to implement a PCIe
> capability.  Generally this is sufficient to assume extended config
> space is present, but we generally also perform additional tests to
> make sure the extended config space is reachable and not simply an
> alias of standard config space.  For a VF to exist extended config
> space must be accessible on the PF, therefore we can also assume it to
> be accessible on the VF.  This enables a micro performance
> optimization previously implemented in commit 975bb8b4dc93 ("PCI/IOV:
> Use VF0 cached config space size for other VFs") to speed up probing
> of VFs.
>
> Cc: KarimAllah Ahmed <karahmed@...zon.de>
> Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> Cc: Hao Zheng <yinhe@...ux.alibaba.com>
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>   drivers/pci/probe.c |   15 +++++++++++++++
>   1 file changed, 15 insertions(+)
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index a3a3c6b28343..439244ff8f09 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1561,6 +1561,21 @@ int pci_cfg_space_size(struct pci_dev *dev)
>   	u32 status;
>   	u16 class;
>   
> +#ifdef CONFIG_PCI_IOV
> +	/*
> +	 * Per the SR-IOV specification (rev 1.1, sec 3.5), VFs are required to
> +	 * implement a PCIe capability and therefore must implement extended
> +	 * config space.  We can skip the NO_EXTCFG test below and the
> +	 * reachability/aliasing test in pci_cfg_space_size_ext() by virtue of
> +	 * the fact that the SR-IOV capability on the PF resides in extended
> +	 * config space and must be accessible and non-aliased to have enabled
> +	 * support for this VF.  This is a micro performance optimization for
> +	 * systems supporting many VFs.
> +	 */
> +	if (dev->is_virtfn)
> +		return PCI_CFG_SPACE_EXP_SIZE;
> +#endif

It looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan 
<sathyanarayanan.kuppuswamy@...ux.intel.com>

> +
>   	if (dev->bus->bus_flags & PCI_BUS_FLAGS_NO_EXTCFG)
>   		return PCI_CFG_SPACE_SIZE;
>   
>
>
-- 
Sathyanarayanan Kuppuswamy
Linux kernel developer