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    SubjectRe: [PATCH 42/70] x86/sev-es: Support nested #VC exceptions
    On Thu, Mar 19, 2020 at 2:14 AM Joerg Roedel <joro@8bytes.org> wrote:
    >
    > From: Joerg Roedel <jroedel@suse.de>
    >
    > Handle #VC exceptions that happen while the GHCB is in use. This can
    > happen when an NMI happens in the #VC exception handler and the NMI
    > handler causes a #VC exception itself. Save the contents of the GHCB
    > when nesting is detected and restore it when the GHCB is no longer
    > used.
    >
    > Signed-off-by: Joerg Roedel <jroedel@suse.de>
    > ---
    > arch/x86/kernel/sev-es.c | 63 +++++++++++++++++++++++++++++++++++++---
    > 1 file changed, 59 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
    > index 97241d2f0f70..3b7bbc8d841e 100644
    > --- a/arch/x86/kernel/sev-es.c
    > +++ b/arch/x86/kernel/sev-es.c
    > @@ -32,9 +32,57 @@ struct ghcb boot_ghcb_page __bss_decrypted __aligned(PAGE_SIZE);
    > */
    > struct ghcb __initdata *boot_ghcb;
    >
    > +struct ghcb_state {
    > + struct ghcb *ghcb;
    > +};
    > +
    > /* Runtime GHCB pointers */
    > static struct ghcb __percpu *ghcb_page;
    >
    > +/*
    > + * Mark the per-cpu GHCB 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.
    > + */
    > +static DEFINE_PER_CPU(bool, ghcb_active);
    > +
    > +static struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
    > +{
    > + struct ghcb *ghcb = (struct ghcb *)this_cpu_ptr(ghcb_page);
    > + bool *active = this_cpu_ptr(&ghcb_active);
    > +
    > + if (unlikely(*active)) {
    > + /* GHCB is already in use - save its contents */
    > +
    > + state->ghcb = kzalloc(sizeof(struct ghcb), GFP_ATOMIC);
    > + if (!state->ghcb)
    > + return NULL;

    This can't possibly end well. Maybe have a little percpu list of
    GHCBs and make sure there are enough for any possible nesting?

    Also, I admit confusion. Isn't the GHCB required to be unencrypted?
    How does that work with kzalloc()?

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