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

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Date:   Mon, 2 Dec 2019 14:19:49 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.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 03:53:15PM -0500, Peter Xu wrote:
> 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. :)

My point is that if the allocation were changed to no longer be a
straightforward alloc_page() then someone reading the combined code would
have no idea why the BUILD_BUG_ON() exists.  It's a bit ridiculous for
this case because the specific constraints of vcpu->run make it highly
unlikely to use anything else, but that's beside the point.

> 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.

But by adding a BUILD_BUG_ON without explaining *why*, you're placing an
extra burden on someone that wants to increase the size of kvm->run, e.g.
it's not at all obvious from the changelog whether this patch is adding
the BUILD_BUG_ON purely because the code allocates memory for vcpu->run
via alloc_page(), or if there is some fundamental aspect of vcpu->run that
requires it to never span multiple pages.