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    SubjectRe: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)
    On Tue, Jun 23, 2020 at 11:45:19AM +0200, Joerg Roedel wrote:
    > Hi Andy,
    >
    > On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
    > > 1. Use IST for #VC and deal with all the mess that entails.
    >
    > With the removal of IST shifting I wonder what you would suggest on how
    > to best implement an NMI-safe IST handler with nesting support.
    >
    > My current plan is to implement an IST handler which switches itself off
    > the IST stack as soon as possible, freeing it for re-use.
    >
    > The flow would be roughly like this upon entering the handler;
    >
    > build_pt_regs();
    >
    > RSP = pt_regs->sp;
    >
    > if (RSP in VC_IST_stack)
    > error("unallowed nesting")
    >
    > if (RSP in current_kernel_stack)
    > RSP = round_down_to_8(RSP)
    > else
    > RSP = current_top_of_stack() // non-ist kernel stack
    >
    > copy_pt_regs(pt_regs, RSP);
    > switch_stack_to(RSP);
    >
    > To make this NMI safe, the NMI handler needs some logic too. Upon
    > entering NMI, it needs to check the return RSP, and if it is in the #VC
    > IST stack, it must do the above flow by itself and update the return RSP
    > and RIP. It needs to take into account the case when PT_REGS is not
    > fully populated on the return side.
    >
    > Alternativly the NMI handler could safe/restore the contents of the #VC
    > IST stack or just switch to a special #VC-in-NMI IST stack.
    >
    > All in all it could get complicated, and imho shift_ist would have been
    > simpler, but who am I anyway...
    >
    > Or maybe you have a better idea how to implement this, so I'd like to
    > hear your opinion first before I spend too many days implementing
    > something.

    OK, excuse my ignorance, but I'm not seeing how that IST shifting
    nonsense would've helped in the first place.

    If I understand correctly the problem is:

    <#VC>
    shift IST
    <NMI>
    ... does stuff
    <#VC> # again, safe because the shift

    But what happens if you get the NMI before your IST adjustment?

    <#VC>
    <NMI>
    ... does stuff
    <#VC> # again, happily wrecks your earlier #VC
    shift IST # whoopsy, too late

    Either way around we get to fix this up in NMI (and any other IST
    exception that can happen while in #VC, hello #MC). And more complexity
    there is the very last thing we need :-(

    There's no way you can fix up the IDT without getting an NMI first.

    This entire exception model is fundamentally buggered :-/

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