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

[Andi Kleen]

On 6/8/2021 10:53 AM, Dave Hansen wrote:
> On 6/8/21 10:48 AM, Sean Christopherson wrote:
> > 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.
> What about tracing?  Can I plop a kprobe in here or turn on ftrace?

I believe neither does mmio/msr normally (except maybe ftrace+tp_printk, 
but that will likely work because it shouldn't recurse more than once 
due to ftrace's reentry protection)

-Andi