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

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Message-ID: <aUL-J-MvdCrCtDp4@google.com>
Date: Wed, 17 Dec 2025 11:01:59 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>
Cc: Chao Gao <chao.gao@...el.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

On Wed, Dec 17, 2025, Xu Yilun wrote:
> > >+#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.

I always forget that the double-underscores have the checks.  

> 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.

I don't have a strong preference.  Using __this_cpu_inc_return() without any
nearby preemption_{enable,disable}() calls makes it quite clears that preemption
is expected to be disabled by the caller.  But I'm also ok being explicit.