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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 04:39:26PM +0100, Andrew Cooper wrote:
    > On 23/06/2020 16:23, Peter Zijlstra wrote:
    > > On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:

    > >> Yes, this is a start, it doesn't cover the case where the NMI stack is
    > >> in-between, so I think you need to walk down regs->sp too.
    > > That shouldn't be possible with the current code, I think.
    >
    > NMI; #MC; Anything which IRET but isn't fatal - #DB, or #BP from
    > patching, #GP from *_safe(), etc; NMI
    >
    > Sure its a corner case, but did you hear that IST is evil?

    Isn't current #MC unconditionally fatal from kernel? But yes, I was
    sorta aware people want that changed.

    And yes, NMI can recurse, mostly on #BP and #PF. Like I wrote, its
    broken vs #MC.

    But Joerg was talking about IST recursion with NMI in the middle,
    something like: #DB, NMI, #DB, and not already being fatal. This one in
    particular is ruled out by #DB itself clearing DR7 (but NMI would also
    do that).

    > P.S. did you also hear that with Rowhammer, userspace has a nonzero
    > quantity of control over generating #MC, depending on how ECC is
    > configured on the platform.

    Yes, excellent stuff.

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