Re: [RFC v2 08/32] x86/traps: Add #VE support for TDX guest

[Sean Christopherson]

On Tue, Jun 08, 2021, Dave Hansen wrote:
> On 4/26/21 11:01 AM, Kuppuswamy Sathyanarayanan wrote:
> > +#ifdef CONFIG_INTEL_TDX_GUEST
> > +DEFINE_IDTENTRY(exc_virtualization_exception)
> > +{
> > +	struct ve_info ve;
> > +	int ret;
> > +
> > +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> > +
> > +	/*
> > +	 * Consume #VE info before re-enabling interrupts. It will be
> > +	 * re-enabled after executing the TDGETVEINFO TDCALL.
> > +	 */
> > +	ret = tdg_get_ve_info(&ve);
> 
> Is it safe to have *anything* before the tdg_get_ve_info()?  For
> instance, say that RCU_LOCKDEP_WARN() triggers.  Will anything in there
> do MMIO?

I doubt it's safe, anything that's doing printing has the potential to trigger
#VE.  Even if we can prove it's safe for all possible paths, I can't think of a
reason to allow anything that's not absolutely necessary before retrieving the
#VE info.