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    SubjectRe: [PATCH RFC] x86/bus_lock: Enable bus lock detection
    On Fri, Jul 17, 2020 at 02:35:00PM -0700, Fenghua Yu wrote:

    > #DB for bus lock detect fixes all issues in #AC for split lock detect:
    > 1) It's architectural ... just need to look at one CPUID bit to know it
    > exists
    > 2) The IA32_DEBUGCTL MSR, which reports bus lock in #DB, is per-thread.
    > So each process or guest can have different behavior.

    And it generates a whole new problem due to #DB being an IST, and

    > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
    > index b038695f36c5..58725567da39 100644
    > --- a/arch/x86/kernel/traps.c
    > +++ b/arch/x86/kernel/traps.c
    > @@ -812,6 +812,16 @@ static void handle_debug(struct pt_regs *regs, unsigned long dr6, bool user)
    > if (!user && !dr6)
    > return;
    >
    > + /* Handle bus lock. */
    > + if (!(dr6 & DR_BUS_LOCK)) {
    > + cond_local_irq_enable(regs);
    > + if (user)
    > + handle_user_bus_lock(regs);
    > + else
    > + handle_kernel_bus_lock(regs);
    > + goto out;
    > + }
    > +
    > /*
    > * If dr6 has no reason to give us about the origin of this trap,
    > * then it's very likely the result of an icebp/int01 trap.

    we very much rely on #DB never recursing, which we carefully crafted by
    disallowing hardare breakpoints on noinstr code and clearing DR7 early.

    But now it can... please keep the pieces.

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