linux-kernel - Re: [PATCH v8 7/9] KVM: Move kvm_arch_vcpu

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Date:   Fri, 15 Apr 2022 15:11:08 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zeng Guang <guang.zeng@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Kai Huang <kai.huang@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Robert Hu <robert.hu@...el.com>,
        Gao Chao <chao.gao@...el.com>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v8 7/9] KVM: Move kvm_arch_vcpu_precreate() under
 kvm->lock

On Fri, Apr 15, 2022, Sean Christopherson wrote:
> > It's safe to invoke kvm_arch_vcpu_precreate() within the protection of
> > kvm->lock directly rather than take into account in the implementation for
> > each architecture.
> 
> This absolutely needs to explain _why_ it's safe, e.g. only arm64, x86, and s390
> have non-nop implementations and they're all simple and short with no tendrils
> into other code that might take kvm->lock.
> 
> And as before, I suspect arm64 needs this protection, the vgic_initialized()
> check looks racy.  Though it's hard to tell if doing the check under kvm->lock
> actually fixes anything.

Ah, I bet this code in vgic_init() provides the necessary protection.

	/* Are we also in the middle of creating a VCPU? */
	if (kvm->created_vcpus != atomic_read(&kvm->online_vcpus))
		return -EBUSY;