[PATCH] ppc64: User tasks must have a valid thread.regs

[Anton Blanchard]

Hi,

> OK, but why wasn't SMP affected?  SMP doesn't have lazy save/restore,
> it should have hit the null regs pointer "sooner".

Because the FPR space is allocated for all tasks in the thread struct:

struct thread_struct {
        unsigned long   ksp;            /* Kernel stack pointer */
        unsigned long   ksp_vsid;
        struct pt_regs  *regs;          /* Pointer to saved register state */
        mm_segment_t    fs;             /* for get_fs() validation */
        double          fpr[32];        /* Complete floating point set */
        unsigned long   fpscr;          /* Floating point status (plus pad) */
        unsigned long   fpexc_mode;     /* Floating-point exception mode */
        unsigned long   pad[3];         /* was saved_msr, saved_softe */
#ifdef CONFIG_ALTIVEC
        /* Complete AltiVec register set */
        vector128       vr[32] __attribute((aligned(16)));
        /* AltiVec status */
        vector128       vscr __attribute((aligned(16)));
        unsigned long   vrsave;
        int             used_vr;        /* set if process has used altivec */
#endif /* CONFIG_ALTIVEC */
};

The SMP lazy FP stuff doesnt touch stuff in thread->regs, but the UP
one does. It hits thread->regs.msr for the previous FP task I think.

Anton