linux-kernel - Re: [PATCH 17/44] KVM: arm64: Do arm/arch initialiation without bouncing through kvm

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Date:   Thu, 3 Nov 2022 15:29:58 +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>,
        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,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Chao Gao <chao.gao@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH 17/44] KVM: arm64: Do arm/arch initialiation without
 bouncing through kvm_init()

On Thu, Nov 03, 2022, Philippe Mathieu-Daudé wrote:
> Hi Sean,
> 
> On 3/11/22 00:18, Sean Christopherson wrote:
> > Move arm/arch specific initialization directly in arm's module_init(),
> > now called kvm_arm_init(), instead of bouncing through kvm_init() to
> > reach kvm_arch_init().  Invoking kvm_arch_init() is the very first action
> > performed by kvm_init(), i.e. this is a glorified nop.
> > 
> > Making kvm_arch_init() a nop will allow dropping it entirely once all
> > other architectures follow suit.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/arm64/kvm/arm.c | 25 ++++++++++++++++---------
> >   1 file changed, 16 insertions(+), 9 deletions(-)
> 
> >   /* NOP: Compiling as a module not supported */
> >   void kvm_arch_exit(void)
> >   {
> > -	kvm_unregister_perf_callbacks();
> 
> Doesn't this belong to the previous patch?

No, but the above changelog is a lie, there is very much a functional change here.

The goal of the previous patch is to fix the error paths in kvm_arch_init(), a.k.a.
kvm_arm_init().  After fixing kvm_arch_init(), there are still bugs in the sequence
as a whole because kvm_arch_exit() doesn't unwind other state, e.g. kvm_arch_exit()
should really look something like:

  void kvm_arch_exit(void)
  {
	teardown_subsystems();

	if (!is_kernel_in_hyp_mode())
		teardown_hyp_mode();

	kvm_arm_vmid_alloc_free();

	if (is_protected_kvm_enabled())
		???	
  }

Becuase although the comment "NOP: Compiling as a module not supported" is correct
about KVM ARM always having to be built into the kernel, kvm_arch_exit() can still
be called if a later stage of kvm_init() fails.

But rather than add a patch to fix kvm_arch_exit(), I chose to fix the bug by
moving code out of kvm_arch_init() so that the unwind sequence established in the
previous patch could be reused.

Except I managed to forget those details when writing the changelog.  The changelog
should instead be:

  KVM: arm64: Do arm/arch initialization without bouncing through kvm_init()

  Do arm/arch specific initialization directly in arm's module_init(), now
  called kvm_arm_init(), instead of bouncing through kvm_init() to reach
  kvm_arch_init().  Invoking kvm_arch_init() is the very first action
  performed by kvm_init(), so from a initialization perspective this is a
  glorified nop.

  Avoiding kvm_arch_init() also fixes a mostly benign bug as kvm_arch_exit()
  doesn't properly unwind if a later stage of kvm_init() fails.  While the
  soon-to-be-deleted comment about compiling as a module being unsupported
  is correct, kvm_arch_exit() can still be called by kvm_init() if any step
  after the call to kvm_arch_init() succeeds.

  Add a FIXME to call out that pKVM initialization isn't unwound if
  kvm_init() fails, which is a pre-existing problem inherited from
  kvm_arch_exit().

  Making kvm_arch_init() a nop will also allow dropping kvm_arch_init() and
  kvm_arch_exit() entirely once all other architectures follow suit.