linux-kernel - Re: [PATCH 39/44] KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm

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Date:   Thu, 3 Nov 2022 17:53:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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 39/44] KVM: Drop kvm_count_lock and instead protect
 kvm_usage_count with kvm_lock

On Thu, Nov 03, 2022, Paolo Bonzini wrote:
> On 11/3/22 00:19, Sean Christopherson wrote:
> > +- kvm_lock is taken outside kvm->mmu_lock
> 
> Not surprising since one is a mutex and one is an rwlock. :)

Heh, 

  Signed-off-by: Captain Obvious <seanjc@...gle.com>

> You can drop this hunk as well as the "Opportunistically update KVM's locking
> documentation" sentence in the commit message.

Will do.

> >   - vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock
> >   - kvm->arch.mmu_lock is an rwlock.  kvm->arch.tdp_mmu_pages_lock and
> > @@ -216,15 +220,11 @@ time it will be set using the Dirty tracking mechanism described above.
> >   :Type:		mutex
> >   :Arch:		any
> >   :Protects:	- vm_list
> > -
> > -``kvm_count_lock``
> > -^^^^^^^^^^^^^^^^^^
> > -
> > -:Type:		raw_spinlock_t
> > -:Arch:		any
> > -:Protects:	- hardware virtualization enable/disable
> > -:Comment:	'raw' because hardware enabling/disabling must be atomic /wrt
> > -		migration.
> > +		- kvm_usage_count
> > +		- hardware virtualization enable/disable
> > +		- module probing (x86 only)
> 
> What do you mean exactly by "module probing"?  Is it anything else than what
> is serialized by vendor_module_lock?

Ooh, I forgot to update this patch after switching to vendor_module_lock.  I
added the above after fixing the first deadlock between kvm_lock and cpu_hotplug_lock,
but later gave up on trying to use kvm_lock after deadlock #2, which is when I
when I realized piggybacking kvm_lock was going to be a maintainance nightmare due.