linux-kernel - Re: TDX #VE in SYSCALL gap (was: [RFD] x86: Curing the exception and syscall trainwreck in hardware)

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Message-ID: <CALCETrUwqka6M9wxkwFYvq-5Byi8JFiiPNRdYQRw3_2m1hXd0w@mail.gmail.com>
Date:   Sun, 30 Aug 2020 08:37:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Pu Wen <puwen@...on.cn>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <alexander.levin@...rosoft.com>,
        Dirk Hohndel <dirkhh@...are.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Tony W Wang-oc <TonyWWang-oc@...oxin.com>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Gordon Tetlow <gordon@...lows.org>,
        David Kaplan <David.Kaplan@....com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: TDX #VE in SYSCALL gap (was: [RFD] x86: Curing the exception and
 syscall trainwreck in hardware)

On Wed, Aug 26, 2020 at 12:16 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Tue, Aug 25, 2020 at 10:28:53AM -0700, Andy Lutomirski wrote:
> > On Tue, Aug 25, 2020 at 10:19 AM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > > One thought would be to have the TDX module (thing that runs in SEAM and
> > > sits between the VMM and the guest) provide a TDCALL (hypercall from guest
> > > to TDX module) to the guest that would allow the guest to specify a very
> > > limited number of GPAs that must never generate a #VE, e.g. go straight to
> > > guest shutdown if a disallowed GPA would go pending.  That seems doable
> > > from a TDX perspective without incurring noticeable overhead (assuming the
> > > list of GPAs is very small) and should be easy to to support in the guest,
> > > e.g. make a TDCALL/hypercall or two during boot to protect the SYSCALL
> > > page and its scratch data.
> >
> > I guess you could do that, but this is getting gross.  The x86
> > architecture has really gone off the rails here.
>
> Does it suck less than using an IST?  Honest question.
>
> I will add my voice to the "fix SYSCALL" train, but the odds of that getting
> a proper fix in time to intercept TDX are not good.  On the other hand,
> "fixing" the SYSCALL issue in the TDX module is much more feasible, but only
> if we see real value in such an approach as opposed to just using an IST.  I
> personally like the idea of a TDX module solution as I think it would be
> simpler for the kernel to implement/support, and would mean we wouldn't need
> to roll back IST usage for #VE if the heavens should part and bestow upon us
> a sane SYSCALL.

There's no such thing as "just" using an IST.  Using IST opens a huge
can of works due to its recursion issues.

The TDX module solution is utterly gross but may well suck less than
using an IST.