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

From: Sean Christopherson <[email-masked]>
Date: Thu, 1 Dec 2022 22:49:06 +0000

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 <[email-masked]>
> > ---
> >   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;