linux-kernel - Re: [PATCH v2 2/7] KVM: x86: Extract VMXON and EFER.SVME enablement to kernel

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Message-ID: <aUJUbcyz2DXmphtU@yilunxu-OptiPlex-7050>
Date: Wed, 17 Dec 2025 14:57:49 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Kiryl Shutsemau <kas@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	linux-coco@...ts.linux.dev, kvm@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 2/7] KVM: x86: Extract VMXON and EFER.SVME enablement
 to kernel

> >+#define x86_virt_call(fn)				\
> >+({							\
> >+	int __r;					\
> >+							\
> >+	if (IS_ENABLED(CONFIG_KVM_INTEL) &&		\
> >+	    cpu_feature_enabled(X86_FEATURE_VMX))	\
> >+		__r = x86_vmx_##fn();			\
> >+	else if (IS_ENABLED(CONFIG_KVM_AMD) &&		\
> >+		 cpu_feature_enabled(X86_FEATURE_SVM))	\
> >+		__r = x86_svm_##fn();			\
> >+	else						\
> >+		__r = -EOPNOTSUPP;			\
> >+							\
> >+	__r;						\
> >+})
> >+
> >+int x86_virt_get_cpu(int feat)
> >+{
> >+	int r;
> >+
> >+	if (!x86_virt_feature || x86_virt_feature != feat)
> >+		return -EOPNOTSUPP;
> >+
> >+	if (this_cpu_inc_return(virtualization_nr_users) > 1)
> >+		return 0;
> 
> Should we assert that preemption is disabled? Calling this API when preemption
> is enabled is wrong.
> 
> Maybe use __this_cpu_inc_return(), which already verifies preemption status.
> 

Is it better we explicitly assert the preemption for x86_virt_get_cpu()
rather than embed the check in __this_cpu_inc_return()? We are not just
protecting the racing for the reference counter. We should ensure the
"counter increase + x86_virt_call(get_cpu)" can't be preempted.

Thanks,
Yilun