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

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Date:   Thu, 24 Mar 2022 09:09:16 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     sthemmin@...rosoft.com, kys@...rosoft.com, haiyangz@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, rafael@...nel.org,
        lenb@...nel.org, lorenzo.pieralisi@....com, robh@...nel.org,
        kw@...ux.com, bhelgaas@...gle.com, hch@....de,
        m.szyprowski@...sung.com, robin.murphy@....com,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v2 2/2] PCI: hv: Propagate coherence from VMbus device to
 PCI device

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@...rosoft.com>

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