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

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Date:   Fri, 12 Feb 2021 12:37:18 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.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>,
        LKML <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

On Fri, Feb 12, 2021, Dave Hansen wrote:
> On 2/12/21 12:06 PM, Sean Christopherson wrote:
> >> What happens if the guest attempts to access a secure GPA that is not
> >> ACCEPTed?  For example, suppose the VMM does THH.MEM.PAGE.REMOVE on a secure
> >> address and the guest accesses it, via instruction fetch or data access.
> >> What happens?
> > Well, as currently written in the spec, it will generate an EPT violation and
> > the host will have no choice but to kill the guest.
> 
> That's actually perfect behavior from my perspective.  Host does
> something stupid.  Host gets left holding the pieces.  No enabling to do
> in the guest.
> 
> This doesn't *preclude* the possibility that the VMM and guest could
> establish a protocol to remove guest pages.  It just means that the host
> can't go it alone and that if they guest and host get out of sync, the
> guest dies.
> 
> In other words, I think I'm rooting for the docs, as written. :)

I tentatively agree that the host should not be able to remove pages without
guest approval, but that's not the only use case for #VE on EPT violations.
It's not even really an intended use case.

There needs to be a mechanism for lazy/deferred/on-demand acceptance of pages.
E.g. pre-accepting every page in a VM with hundreds of GB of memory will be
ridiculously slow.

#VE is the best option to do that:

  - Relatively sane re-entrancy semantics.
  - Hardware accelerated.
  - Doesn't require stealing an IRQ from the guest.