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

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Message-ID: <20200325164606.GH14294@linux.intel.com>
Date:   Wed, 25 Mar 2020 09:46:06 -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 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.

Assuming all issues can be fixed, I think it'd make sense to keep the
BUILD_BUG, especially if it's teasing out actual weirdness, even if the
weirdness is benign.