On Mon, Sep 18, 2023, Jason Gunthorpe wrote:

On Fri, Sep 15, 2023 at 05:30:58PM -0700, Sean Christopherson wrote:

quoted

Drop KVM's KVM_VFIO Kconfig, and instead compile in VFIO support if
and only if VFIO itself is enabled.  Similar to the recent change to have
VFIO stop looking at HAVE_KVM, compiling in support for talking to VFIO
just because the architecture supports VFIO is nonsensical.

This fixes a bug where RISC-V doesn't select KVM_VFIO, i.e. would silently
fail to do connect KVM and VFIO, even though RISC-V supports VFIO.  The
bug is benign as the only driver in all of Linux that actually uses the
KVM reference provided by VFIO is KVM-GT, which is x86/Intel specific.

Hmm, I recall that all the S390 drivers need it as well.

static int vfio_ap_mdev_open_device(struct vfio_device *vdev)
{
        struct ap_matrix_mdev *matrix_mdev =
                container_of(vdev, struct ap_matrix_mdev, vdev);

        if (!vdev->kvm)
                return -EINVAL;

        return vfio_ap_mdev_set_kvm(matrix_mdev, vdev->kvm);

Ah, I missed that the KVM reference was routed through VFIO in that case.

I wonder if we should be making the VFIO drivers that need the kvm to
ask for it? 'select CONFIG_NEED_VFIO_KVM' or something?

I wondered the same thing, if only to make it easier to track which drivers actually
end up interacting directly with KVM.

Regardless, I fully agree with getting rid of the arch flag.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

quoted

--- a/virt/kvm/Makefile.kvm
+++ b/virt/kvm/Makefile.kvm

@@ -6,7 +6,9 @@
 KVM ?= ../../../virt/kvm
 
 kvm-y := $(KVM)/kvm_main.o $(KVM)/eventfd.o $(KVM)/binary_stats.o
-kvm-$(CONFIG_KVM_VFIO) += $(KVM)/vfio.o
+ifdef CONFIG_VFIO
+kvm-y += $(KVM)/vfio.o
+endif

I wonder if kvm-m magically works in kbuild so you don't need the ifdef?

Yeah, that should work, no idea why I added the ifdef.

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