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Date:   Fri, 15 Apr 2022 15:11:08 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Zeng Guang <guang.zeng@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Kim Phillips <kim.phillips@amd.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jethro Beekman <jethro@fortanix.com>,
        Kai Huang <kai.huang@intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Robert Hu <robert.hu@intel.com>,
        Gao Chao <chao.gao@intel.com>, Marc Zyngier <maz@kernel.org>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v8 7/9] KVM: Move kvm_arch_vcpu_precreate() under
 kvm->lock
Message-ID: <YlmLDFPtQeI5wIoI@google.com>
References: <20220411090447.5928-1-guang.zeng@intel.com>
 <20220411090447.5928-8-guang.zeng@intel.com>
 <YlmIko9PbQMMKceU@google.com>
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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;