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    SubjectRe: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)
    Hi Peter,

    On Tue, Jun 23, 2020 at 12:45:59PM +0200, Peter Zijlstra wrote:
    > On Tue, Jun 23, 2020 at 11:45:19AM +0200, Joerg Roedel wrote:
    > > 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?

    The v3 patchset implements an unconditional shift of the #VC IST entry
    in the NMI handler, before it can trigger a #VC exception.

    > 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 :-(

    Yes, in whatever way this gets implemented, it needs some fixup in the
    NMI handler. But that can happen in C code, so it does not make the
    assembly more complex, at least.

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

    Not sure what you mean by this.

    > This entire exception model is fundamentally buggered :-/



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