Re: [PATCH v8 7/9] KVM: Move kvm_arch_vcpu_precreate() under kvm->lock

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
Date: Fri, 15 Apr 2022 15:11:08 +0000	[thread overview]
Message-ID: <YlmLDFPtQeI5wIoI@google.com> (raw)
In-Reply-To: <YlmIko9PbQMMKceU@google.com>

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;