linux-kernel - Re: [PATCH 06/26] KVM: Drop CONFIG_KVM

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Message-ID: <ZQhySMjmuyW2Czas@google.com>
Date:   Mon, 18 Sep 2023 08:52:40 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
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        Alex Williamson <alex.williamson@...hat.com>,
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        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
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        Anish Ghulati <aghulati@...gle.com>,
        Venkatesh Srinivas <venkateshs@...omium.org>,
        Andrew Thornton <andrewth@...gle.com>
Subject: Re: [PATCH 06/26] KVM: Drop CONFIG_KVM_VFIO and just look at KVM+VFIO

On Mon, Sep 18, 2023, Jason Gunthorpe wrote:
> On Fri, Sep 15, 2023 at 05:30:58PM -0700, Sean Christopherson wrote:
> > 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@...dia.com>
> 
> > --- 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.