linux-kernel - Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest

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Message-ID: <20210208162301.GA365765@tassilo.jf.intel.com>
Date:   Mon, 8 Feb 2021 08:23:01 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest

> 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.

> > +#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.


-Andi