linux-kernel - Re: linux-next: Tree for Mar 25 (arch/x86/kvm/)

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Date:   Wed, 25 Mar 2020 11:47:41 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        KVM <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: linux-next: Tree for Mar 25 (arch/x86/kvm/)

On Wed, Mar 25, 2020 at 09:46:06AM -0700, Sean Christopherson wrote:
> On Wed, Mar 25, 2020 at 05:26:20PM +0100, Paolo Bonzini wrote:
> > On 25/03/20 17:14, Sean Christopherson wrote:
> > >> Doh, right.  I think the only solution for that one is to degrade it to
> > >> WARN_ON(1).
> > > I reproduced the error, give me a bit to play with the code to see if the
> > > BUILD_BUG can be preserved.  I'm curious as to why kvm_cpu_cap_mask() is
> > > special, and why it only fails with this config.
> > > 
> > 
> > I could not reproduce it, but I would not be surprised if there are
> > other configurations where the compiler cannot constant-propagate from
> > the reverse_cpuid struct into __cpuid_entry_get_reg.
> 
> The error is related to UBSAN.  There is at least one legitimate (but benign)
> underlying issue.  I'm chasing down a second instance of the BUILD_BUG.

Argh, red herring.  There is no underlying issue other than gcc tripping up
when -fsanitize=alignment is enabled by UBSAN.  Good news is that the build
error can be fixed without resorting to a hack.