linux-kernel - Re: [PATCH v2 2/3] KVM: x86: Allocate kvm_vmx/kvm

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Message-ID: <aAALoMbz0IZcKZk4@google.com>
Date: Wed, 16 Apr 2025 12:57:20 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Vipin Sharma <vipinsh@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] KVM: x86: Allocate kvm_vmx/kvm_svm structures
 using kzalloc()

On Wed, Apr 16, 2025, Vipin Sharma wrote:
> On 2025-04-16 11:24:37, Vipin Sharma wrote:
> > On 2025-04-01 08:57:13, Sean Christopherson wrote:
> > >  
> > > +	BUILD_BUG_ON(get_order(sizeof(struct kvm_svm) != 0));
> > 
> > There is a typo here. It is checking sizeof(struct kvm_svm) != 0, instead
> > of checking get_order(...) != 0.
> > 
> > >  	return 0;
> > >  
> > >  err_kvm_init:
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index b70ed72c1783..01264842bf45 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -8755,6 +8755,7 @@ static int __init vmx_init(void)
> > >  	if (r)
> > >  		goto err_kvm_init;
> > >  
> > > +	BUILD_BUG_ON(get_order(sizeof(struct kvm_vmx) != 0));
> > 
> > Same as above.

Ugh.  That's what I get for violating the kernel's "don't check for '0'" rule
(I thought it would make the code more understandable).  Bad me.

> After fixing the typo build is failing.
> 
> Checked via pahole, sizes of struct have reduced but still not under 4k.
> After applying the patch:
> 
> struct kvm{} - 4104
> struct kvm_svm{} - 4320
> struct kvm_vmx{} - 4128
> 
> Also, this BUILD_BUG_ON() might not be reliable unless all of the ifdefs
> under kvm_[vmx|svm] and its children are enabled. Won't that be an
> issue?

That's what build bots (and to a lesser extent, maintainers) are for.  An individual
developer might miss a particular config, but the build bots that run allyesconfig
will very quickly detect the issue, and then we fix it.

I also build what is effectively an "allkvmconfig" before officially applying
anything, so in general things like this shouldn't even make it to the bots.