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

From: Robin Murphy <robin.murphy@arm.com>
To: Michael Kelley <mikelley@microsoft.com>,
	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, 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 12:23:33 +0000	[thread overview]
Message-ID: <e6d9af22-df69-bf6a-7877-b916918d0682@arm.com> (raw)
In-Reply-To: <1648067472-13000-3-git-send-email-mikelley@microsoft.com>

On 2022-03-23 20:31, 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.

Honestly, I don't hate this :)

It seems conceptually accurate, as far as I understand, and in 
functional terms I'm starting to think it might even be the most correct 
approach anyway. In the physical world we might be surprised to find the 
PCI side of a host bridge behind anything other than some platform/ACPI 
device representing the other side of a physical host bridge or root 
complex, but who's to say that a paravirtual world can't present a more 
abstract topology? Either way, a one-line way of tying in to the 
standard flow is hard to turn down.

Acked-by: Robin Murphy <robin.murphy@arm.com>

> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> ---
>   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;