linux-kernel - Re: [PATCH v2 21/50] KVM: MIPS: Hardcode callbacks to hardware virtualization extensions

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Date:   Thu, 1 Dec 2022 22:49:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Philippe Mathieu-Daudé <philmd@...aro.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Eric Farman <farman@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Paul Durrant <paul@....org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Atish Patra <atishp@...shpatra.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, Yuan Yao <yuan.yao@...el.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Kai Huang <kai.huang@...el.com>, Chao Gao <chao.gao@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 21/50] KVM: MIPS: Hardcode callbacks to hardware
 virtualization extensions

On Thu, Dec 01, 2022, Philippe Mathieu-Daudé wrote:
> On 1/12/22 00:09, Sean Christopherson wrote:
> > Now that KVM no longer supports trap-and-emulate (see commit 45c7e8af4a5e
> > "MIPS: Remove KVM_TE support"), hardcode the MIPS callbacks to the
> > virtualization callbacks.
> > 
> > Harcoding the callbacks eliminates the technically-unnecessary check on
> > non-NULL kvm_mips_callbacks in kvm_arch_init().  MIPS has never supported
> > multiple in-tree modules, i.e. barring an out-of-tree module, where
> > copying and renaming kvm.ko counts as "out-of-tree", KVM could never
> > encounter a non-NULL set of callbacks during module init.
> > 
> > The callback check is also subtly broken, as it is not thread safe,
> > i.e. if there were multiple modules, loading both concurrently would
> > create a race between checking and setting kvm_mips_callbacks.
> > 
> > Given that out-of-tree shenanigans are not the kernel's responsibility,
> > hardcode the callbacks to simplify the code.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/mips/include/asm/kvm_host.h |  2 +-
> >   arch/mips/kvm/Makefile           |  2 +-
> >   arch/mips/kvm/callback.c         | 14 --------------
> >   arch/mips/kvm/mips.c             |  9 ++-------
> >   arch/mips/kvm/vz.c               |  7 ++++---
> >   5 files changed, 8 insertions(+), 26 deletions(-)
> >   delete mode 100644 arch/mips/kvm/callback.c
> > 
> > diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h
> > index 28f0ba97db71..2803c9c21ef9 100644
> > --- a/arch/mips/include/asm/kvm_host.h
> > +++ b/arch/mips/include/asm/kvm_host.h
> > @@ -758,7 +758,7 @@ struct kvm_mips_callbacks {
> >   	void (*vcpu_reenter)(struct kvm_vcpu *vcpu);
> >   };
> >   extern struct kvm_mips_callbacks *kvm_mips_callbacks;
> 
> IIUC we could even constify this pointer.

Good point.  Protecting the pointer itself is a bit gross, but it is a nice
stopgap until the callbacks are gone.  I'll fold this in.  Thanks!

  extern const struct kvm_mips_callbacks * const kvm_mips_callbacks;