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    SubjectRe: [PATCH v4 00/75] x86: SEV-ES Guest Support
    On Wed, Jul 15, 2020 at 11:34:26AM +0200, Joerg Roedel wrote:
    > On Wed, Jul 15, 2020 at 11:24:56AM +0200, Peter Zijlstra wrote:
    > > Can we get some more words -- preferably in actual code comments, on
    > > when exactly #VC happens?
    >
    > Sure, will add this as a comment before the actual runtime VC handler.

    Thanks!

    > > Because the only thing I remember is that #VC could happen on any memop,
    > > but I also have vague memories of that being a later extention.
    >
    > Currently it is only raised when something happens that the hypervisor
    > intercepts, for example on a couple of instructions like CPUID,
    > RD/WRMSR, ..., or on MMIO/IOIO accesses.
    >
    > With Secure Nested Paging (SNP), which needs additional enablement, a #VC can
    > happen on any memory access. I wrote the IST handling entry code for #VC
    > with that in mind, but do not actually enable it. This is the reason why
    > the #VC handler just panics the system when it ends up on the fall-back
    > (VC2) stack, with SNP enabled it needs to handle the SNP exit-codes in
    > that path.

    And recursive #VC was instant death, right? Because there's no way to
    avoid IST stack corruption in that case.

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