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From:   Haiyang Zhang <haiyangz@microsoft.com>
To:     vkuznets <vkuznets@redhat.com>,
        "sashal@kernel.org" <sashal@kernel.org>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "saeedm@mellanox.com" <saeedm@mellanox.com>,
        "leon@kernel.org" <leon@kernel.org>,
        "eranbe@mellanox.com" <eranbe@mellanox.com>,
        "lorenzo.pieralisi@arm.com" <lorenzo.pieralisi@arm.com>,
        "bhelgaas@google.com" <bhelgaas@google.com>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>
CC:     KY Srinivasan <kys@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH net-next, 2/6] PCI: hv: Add a Hyper-V PCI mini driver for
 software backchannel interface
Thread-Topic: [PATCH net-next, 2/6] PCI: hv: Add a Hyper-V PCI mini driver for
 software backchannel interface
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Date:   Fri, 16 Aug 2019 14:48:48 +0000
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> -----Original Message-----
> From: Vitaly Kuznetsov <vkuznets@redhat.com>
> Sent: Friday, August 16, 2019 8:28 AM
> To: Haiyang Zhang <haiyangz@microsoft.com>; sashal@kernel.org;
> davem@davemloft.net; saeedm@mellanox.com; leon@kernel.org;
> eranbe@mellanox.com; lorenzo.pieralisi@arm.com; bhelgaas@google.com;
> linux-pci@vger.kernel.org; linux-hyperv@vger.kernel.org;
> netdev@vger.kernel.org
> Cc: Haiyang Zhang <haiyangz@microsoft.com>; KY Srinivasan
> <kys@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> linux-kernel@vger.kernel.org
> Subject: Re: [PATCH net-next, 2/6] PCI: hv: Add a Hyper-V PCI mini driver=
 for
> software backchannel interface
>=20
> Haiyang Zhang <haiyangz@microsoft.com> writes:
>=20
> > This mini driver is a helper driver allows other drivers to have a
> > common interface with the Hyper-V PCI frontend driver.
> >
> > Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
> > Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
> > ---
> >  MAINTAINERS                              |  1 +
> >  drivers/pci/Kconfig                      |  1 +
> >  drivers/pci/controller/Kconfig           |  7 ++++
> >  drivers/pci/controller/Makefile          |  1 +
> >  drivers/pci/controller/pci-hyperv-mini.c | 70
> ++++++++++++++++++++++++++++++++
> >  drivers/pci/controller/pci-hyperv.c      | 12 ++++--
> >  include/linux/hyperv.h                   | 30 ++++++++++----
> >  7 files changed, 111 insertions(+), 11 deletions(-)  create mode
> > 100644 drivers/pci/controller/pci-hyperv-mini.c
> >
> > diff --git a/MAINTAINERS b/MAINTAINERS index e352550..c4962b9 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -7453,6 +7453,7 @@ F:	drivers/hid/hid-hyperv.c
> >  F:	drivers/hv/
> >  F:	drivers/input/serio/hyperv-keyboard.c
> >  F:	drivers/pci/controller/pci-hyperv.c
> > +F:	drivers/pci/controller/pci-hyperv-mini.c
> >  F:	drivers/net/hyperv/
> >  F:	drivers/scsi/storvsc_drv.c
> >  F:	drivers/uio/uio_hv_generic.c
> > diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig index
> > 2ab9240..bb852f5 100644
> > --- a/drivers/pci/Kconfig
> > +++ b/drivers/pci/Kconfig
> > @@ -182,6 +182,7 @@ config PCI_LABEL
> >  config PCI_HYPERV
> >          tristate "Hyper-V PCI Frontend"
> >          depends on X86 && HYPERV && PCI_MSI && PCI_MSI_IRQ_DOMAIN
> &&
> > X86_64
> > +	select PCI_HYPERV_MINI
> >          help
> >            The PCI device frontend driver allows the kernel to import a=
rbitrary
> >            PCI devices from a PCI backend to support PCI driver domains=
.
> > diff --git a/drivers/pci/controller/Kconfig
> > b/drivers/pci/controller/Kconfig index fe9f9f1..8e31cba 100644
> > --- a/drivers/pci/controller/Kconfig
> > +++ b/drivers/pci/controller/Kconfig
> > @@ -281,5 +281,12 @@ config VMD
> >  	  To compile this driver as a module, choose M here: the
> >  	  module will be called vmd.
> >
> > +config PCI_HYPERV_MINI
> > +	tristate "Hyper-V PCI Mini"
> > +	depends on X86 && HYPERV && PCI_MSI && PCI_MSI_IRQ_DOMAIN
> && X86_64
> > +	help
> > +	  The Hyper-V PCI Mini is a helper driver allows other drivers to
> > +	  have a common interface with the Hyper-V PCI frontend driver.
> > +
>=20
> Out of pure curiosity, why not just export this interface from PCI_HYPERV
> directly? Why do we need this stub?

The pci_hyperv can only be loaded on VMs on Hyper-V and Azure. Other=20
drivers like MLX5e will have symbolic dependency of pci_hyperv if they=20
use functions exported by pci_hyperv. This dependency will cause other=20
drivers fail to load on other platforms, like VMs on KVM. So we created=20
this mini driver, which can be loaded on any platforms to provide the=20
symbolic dependency.

Thanks,
- Haiyang