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    SubjectRe: [PATCH v4 3/6] x86/sev-es: Split up runtime #VC handler for correct state tracking

    Bah, I suppose the trouble is that this SEV crap requires PARAVIRT?

    I should really get around to fixing noinstr validation with PARAVIRT on
    :-(

    On Thu, Jun 10, 2021 at 11:11:38AM +0200, Joerg Roedel wrote:

    > +static void vc_handle_from_kernel(struct pt_regs *regs, unsigned long error_code)

    static noinstr ...

    > +{
    > + irqentry_state_t irq_state = irqentry_nmi_enter(regs);
    >
    > + instrumentation_begin();
    >
    > + if (!vc_raw_handle_exception(regs, error_code)) {
    > /* Show some debug info */
    > show_regs(regs);
    >
    > @@ -1434,7 +1400,59 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
    > panic("Returned from Terminate-Request to Hypervisor\n");
    > }
    >
    > + instrumentation_end();
    > + irqentry_nmi_exit(regs, irq_state);
    > +}
    > +
    > +static void vc_handle_from_user(struct pt_regs *regs, unsigned long error_code)

    static noinstr ...

    > +{
    > + irqentry_state_t irq_state = irqentry_enter(regs);
    > +
    > + instrumentation_begin();
    > +
    > + if (!vc_raw_handle_exception(regs, error_code)) {
    > + /*
    > + * Do not kill the machine if user-space triggered the
    > + * exception. Send SIGBUS instead and let user-space deal with
    > + * it.
    > + */
    > + force_sig_fault(SIGBUS, BUS_OBJERR, (void __user *)0);
    > + }
    > +
    > + instrumentation_end();
    > + irqentry_exit(regs, irq_state);
    > +}

    + linebreak

    > +/*
    > + * Main #VC exception handler. It is called when the entry code was able to
    > + * switch off the IST to a safe kernel stack.
    > + *
    > + * With the current implementation it is always possible to switch to a safe
    > + * stack because #VC exceptions only happen at known places, like intercepted
    > + * instructions or accesses to MMIO areas/IO ports. They can also happen with
    > + * code instrumentation when the hypervisor intercepts #DB, but the critical
    > + * paths are forbidden to be instrumented, so #DB exceptions currently also
    > + * only happen in safe places.
    > + */
    > +DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
    > +{
    > + /*
    > + * Handle #DB before calling into !noinstr code to avoid recursive #DB.
    > + */
    > + if (error_code == SVM_EXIT_EXCP_BASE + X86_TRAP_DB) {
    > + vc_handle_trap_db(regs);
    > + return;
    > + }
    > +
    > + /*
    > + * This is invoked through an interrupt gate, so IRQs are disabled. The
    > + * code below might walk page-tables for user or kernel addresses, so
    > + * keep the IRQs disabled to protect us against concurrent TLB flushes.
    > + */
    > +
    > + if (user_mode(regs))
    > + vc_handle_from_user(regs, error_code);
    > + else
    > + vc_handle_from_kernel(regs, error_code);
    > }

    #DB and MCE use idtentry_mce_db and split out in asm. When I look at
    idtentry_vc, it appears to me that VC_SAFE_STACK already implies
    from-user, or am I reading that wrong?

    Ah, it appears you're muddling things up again by then also calling
    safe_stack_ from exc_.

    How about you don't do that and have exc_ call your new from_kernel
    function, then we know that safe_stack_ is always from-user. Then also
    maybe do:

    s/VS_SAFE_STACK/VC_USER/
    s/safe_stack_/noist_/

    to match all the others (#DB/MCE).

    Also, AFAICT, you don't actually need DEFINE_IDTENTRY_VC_IST, it doesn't
    have an ASM counterpart.

    So then you end up with something like:

    DEFINE_IDTENTRY_VC(exc_vc)
    {
    if (unlikely(on_vc_fallback_stack(regs))) {
    instrumentation_begin();
    panic("boohooo\n");
    instrumentation_end();
    }

    vc_from_kernel();
    }

    DEFINE_IDTENTRY_VC_USER(exc_vc)
    {
    vc_from_user();
    }

    Which is, I'm thinking, much simpler, no?

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