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Date:   Mon, 8 Feb 2021 17:33:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        linux-kernel@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest
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On Mon, Feb 08, 2021 at 08:23:01AM -0800, Andi Kleen wrote:
> > Which is supposedly then set up to avoid #VE during the syscall gap,
> > yes? Which then results in #VE not having to be IST.
> 
> Yes that is currently true because all memory is pre-accepted.
> 
> If we ever do lazy accept we would need to make sure the memory accessed in
> the syscall gap is already accepted, or move over to an IST.

I think we're going to mandate the entry text/data will have to be
pre-accepted to avoid IST. ISTs really are crap.

> > > +#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 */
> > 
> > So what happens if NMI happens here, and triggers a nested #VE ?
> 
> Yes that's a gap. We should probably bail out and reexecute the original
> instruction. The VE handler would need to set a flag for that.
> 
> Or alternatively the NMI always gets the VE information and puts
> it on some internal stack, but that would seem clunkier.

The same is possible with MCE and #DB I imagine.