Sometimes when setting a breakpoint a process doesn't stop on it.
This is because the debug registers are not loaded correctly on
VCPU load.

The following simple reproducer from Oleg Nesterov tries using debug
registers in both the host and the guest, for example by running "./bp
0 1" on the host and "./bp 14 15" under QEMU.

    #include <unistd.h>
    #include <signal.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <sys/wait.h>
    #include <sys/ptrace.h>
    #include <sys/user.h>
    #include <asm/debugreg.h>
    #include <assert.h>

    #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

    unsigned long encode_dr7(int drnum, int enable, unsigned int type, unsigned int len)
    {
        unsigned long dr7;

        dr7 = ((len | type) & 0xf)
            << (DR_CONTROL_SHIFT + drnum * DR_CONTROL_SIZE);
        if (enable)
            dr7 |= (DR_GLOBAL_ENABLE << (drnum * DR_ENABLE_SIZE));

        return dr7;
    }

    int write_dr(int pid, int dr, unsigned long val)
    {
        return ptrace(PTRACE_POKEUSER, pid,
                offsetof (struct user, u_debugreg[dr]),
                val);
    }

    void set_bp(pid_t pid, void *addr)
    {
        unsigned long dr7;
        assert(write_dr(pid, 0, (long)addr) == 0);
        dr7 = encode_dr7(0, 1, DR_RW_EXECUTE, DR_LEN_1);
        assert(write_dr(pid, 7, dr7) == 0);
    }

    void *get_rip(int pid)
    {
        return (void*)ptrace(PTRACE_PEEKUSER, pid,
                offsetof(struct user, regs.rip), 0);
    }

    void test(int nr)
    {
        void *bp_addr = &&label + nr, *bp_hit;
        int pid;

        printf("test bp %d\n", nr);
        assert(nr < 16); // see 16 asm nops below

        pid = fork();
        if (!pid) {
            assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
            kill(getpid(), SIGSTOP);
            for (;;) {
                label: asm (
                    "nop; nop; nop; nop;"
                    "nop; nop; nop; nop;"
                    "nop; nop; nop; nop;"
                    "nop; nop; nop; nop;"
                );
            }
        }

        assert(pid == wait(NULL));
        set_bp(pid, bp_addr);

        for (;;) {
            assert(ptrace(PTRACE_CONT, pid, 0, 0) == 0);
            assert(pid == wait(NULL));

            bp_hit = get_rip(pid);
            if (bp_hit != bp_addr)
                fprintf(stderr, "ERR!! hit wrong bp %ld != %d\n",
                    bp_hit - &&label, nr);
        }
    }

    int main(int argc, const char *argv[])
    {
        while (--argc) {
            int nr = atoi(*++argv);
            if (!fork())
                test(nr);
        }

        while (wait(NULL) > 0)
            ;
        return 0;
    }

Cc: stable@vger.kernel.org
Suggested-by: Nadadv Amit <namit@cs.technion.ac.il>
Reported-by: Andrey Wagin <avagin@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>