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    SubjectRe: [PATCH v3 47/75] x86/sev-es: Add Runtime #VC Exception Handler
    On Sat, May 23, 2020 at 09:59:24AM +0200, Borislav Petkov wrote:
    > On Tue, Apr 28, 2020 at 05:16:57PM +0200, Joerg Roedel wrote:
    > > + /*
    > > + * Mark the per-cpu GHCBs as in-use to detect nested #VC exceptions.
    > > + * There is no need for it to be atomic, because nothing is written to
    > > + * the GHCB between the read and the write of ghcb_active. So it is safe
    > > + * to use it when a nested #VC exception happens before the write.
    > > + */
    >
    > Looks liks that is that text... support for nested #VC exceptions.
    > I'm sure this has come up already but why do we even want to support
    > nested #VCs? IOW, can we do without them first or are they absolutely
    > necessary?
    >
    > I'm guessing VC exceptions inside the VC handler but what are the
    > sensible use cases?

    The most important use-case is #VC->NMI->#VC. When an NMI hits while the
    #VC handler uses the GHCB and the NMI handler causes another #VC, then
    the contents of the GHCB needs to be backed up, so that it doesn't
    destroy the GHCB contents of the first #VC handling path.


    Regards,

    Joerg

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