Re: [PATCH v3 05/16] x86/virt: Force-clear X86_FEATURE_VMX if configuring root VMCS fails

[Sean Christopherson]

On Mon, Feb 16, 2026, dan.j.williams@xxxxxxxxx wrote:
> Sean Christopherson wrote:
> > If allocating and configuring a root VMCS fails, clear X86_FEATURE_VMX in
> > all CPUs so that KVM doesn't need to manually check root_vmcs.  As added
> > bonuses, clearing VMX will reflect that VMX is unusable in /proc/cpuinfo,
> > and will avoid a futile auto-probe of kvm-intel.ko.
> > 
> > WARN if allocating a root VMCS page fails, e.g. to help users figure out
> > why VMX is broken in the unlikely scenario something goes sideways during
> > boot (and because the allocation should succeed unless there's a kernel
> > bug).  Tweak KVM's error message to suggest checking kernel logs if VMX is
> > unsupported (in addition to checking BIOS).
> > 
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> [..]
> > diff --git a/arch/x86/virt/hw.c b/arch/x86/virt/hw.c
> > index 56972f594d90..40495872fdfb 100644
> > --- a/arch/x86/virt/hw.c
> > +++ b/arch/x86/virt/hw.c
> [..]
> > @@ -56,7 +56,7 @@ static __init int x86_vmx_init(void)
> >  		struct vmcs *vmcs;
> >  
> >  		page = __alloc_pages_node(node, GFP_KERNEL | __GFP_ZERO, 0);
> > -		if (!page) {
> > +		if (WARN_ON_ONCE(!page)) {
> 
> Is the warn_alloc() deep in this path not sufficient? Either way, this
> patch looks good to me.

Not sure, I don't have much experience with warn_alloc() in practice.  Reading
the code, my initial reaction is that I don't want to rely on warn_alloc() since
it's ratelimited.  Multiple allocation failures during boot seems unlikely, but
at the same time, the cost of the WARN_ON_ONCE() here is really just the handful
of bytes for the bug_table entry.