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

[Sean Christopherson]

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.