linux-kernel - Re: [PATCH RFC 03/15] KVM: Add build-time error check on kvm

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Message-ID: <20191202205315.GD31681@xz-x1>
Date:   Mon, 2 Dec 2019 15:53:15 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 03/15] KVM: Add build-time error check on kvm_run size

On Mon, Dec 02, 2019 at 11:30:27AM -0800, Sean Christopherson wrote:
> On Fri, Nov 29, 2019 at 04:34:53PM -0500, Peter Xu wrote:
> > It's already going to reach 2400 Bytes (which is over half of page
> > size on 4K page archs), so maybe it's good to have this build-time
> > check in case it overflows when adding new fields.
> 
> Please explain why exceeding PAGE_SIZE is a bad thing.  I realize it's
> almost absurdly obvious when looking at the code, but a) the patch itself
> does not provide that context and b) the changelog should hold up on its
> own,

Right, I'll enhance the commit message.

> e.g. in a mostly hypothetical case where the allocation of vcpu->run
> were changed to something else.

And that's why I added BUILD_BUG_ON right beneath that allocation. :)

It's just a helper for developers when adding new kvm_run fields, not
a risk for anyone who wants to start allocating more pages for it.

Thanks,

> 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >  virt/kvm/kvm_main.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 8f8940cc4b84..681452d288cd 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -352,6 +352,8 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
> >  	}
> >  	vcpu->run = page_address(page);
> >  
> > +	BUILD_BUG_ON(sizeof(struct kvm_run) > PAGE_SIZE);
> > +
> >  	kvm_vcpu_set_in_spin_loop(vcpu, false);
> >  	kvm_vcpu_set_dy_eligible(vcpu, false);
> >  	vcpu->preempted = false;
> > -- 
> > 2.21.0
> > 
> 

-- 
Peter Xu