Re: [PATCH v2 2/2] PCI: hv: Propagate coherence from VMbus device to PCI device

From: Boqun Feng <boqun.feng@gmail.com>
To: Michael Kelley <mikelley@microsoft.com>
Cc: sthemmin@microsoft.com, kys@microsoft.com,
	haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com,
	rafael@kernel.org, lenb@kernel.org, lorenzo.pieralisi@arm.com,
	robh@kernel.org, kw@linux.com, bhelgaas@google.com, hch@lst.de,
	m.szyprowski@samsung.com, robin.murphy@arm.com,
	linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-hyperv@vger.kernel.org, linux-pci@vger.kernel.org,
	iommu@lists.linux-foundation.org
Subject: Re: [PATCH v2 2/2] PCI: hv: Propagate coherence from VMbus device to PCI device
Date: Thu, 24 Mar 2022 09:09:16 +0800	[thread overview]
Message-ID: <YjvEvFFuKJiV/NU+@boqun-archlinux> (raw)
In-Reply-To: <1648067472-13000-3-git-send-email-mikelley@microsoft.com>

On Wed, Mar 23, 2022 at 01:31:12PM -0700, Michael Kelley wrote:
> PCI pass-thru devices in a Hyper-V VM are represented as a VMBus
> device and as a PCI device.  The coherence of the VMbus device is
> set based on the VMbus node in ACPI, but the PCI device has no
> ACPI node and defaults to not hardware coherent.  This results
> in extra software coherence management overhead on ARM64 when
> devices are hardware coherent.
> 
> Fix this by setting up the PCI host bus so that normal
> PCI mechanisms will propagate the coherence of the VMbus
> device to the PCI device. There's no effect on x86/x64 where
> devices are always hardware coherent.
> 
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>

Acked-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> ---
>  drivers/pci/controller/pci-hyperv.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index ae0bc2f..88b3b56 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -3404,6 +3404,15 @@ static int hv_pci_probe(struct hv_device *hdev,
>  	hbus->bridge->domain_nr = dom;
>  #ifdef CONFIG_X86
>  	hbus->sysdata.domain = dom;
> +#elif defined(CONFIG_ARM64)
> +	/*
> +	 * Set the PCI bus parent to be the corresponding VMbus
> +	 * device. Then the VMbus device will be assigned as the
> +	 * ACPI companion in pcibios_root_bridge_prepare() and
> +	 * pci_dma_configure() will propagate device coherence
> +	 * information to devices created on the bus.
> +	 */
> +	hbus->sysdata.parent = hdev->device.parent;
>  #endif
>  
>  	hbus->hdev = hdev;
> -- 
> 1.8.3.1
> 
>