• Paul Mackerras's avatar
    powerpc: Don't corrupt transactional state when using FP/VMX in kernel · d31626f7
    Paul Mackerras authored
    Currently, when we have a process using the transactional memory
    facilities on POWER8 (that is, the processor is in transactional
    or suspended state), and the process enters the kernel and the
    kernel then uses the floating-point or vector (VMX/Altivec) facility,
    we end up corrupting the user-visible FP/VMX/VSX state.  This
    happens, for example, if a page fault causes a copy-on-write
    operation, because the copy_page function will use VMX to do the
    copy on POWER8.  The test program below demonstrates the bug.
    
    The bug happens because when FP/VMX state for a transactional process
    is stored in the thread_struct, we store the checkpointed state in
    .fp_state/.vr_state and the transactional (current) state in
    .transact_fp/.transact_vr.  However, when the kernel wants to use
    FP/VMX, it calls enable_kernel_fp() or enable_kernel_altivec(),
    which saves the current state in .fp_state/.vr_state.  Furthermore,
    when we return to the user process we return with FP/VMX/VSX
    disabled.  The next time the process uses FP/VMX/VSX, we don't know
    which set of state (the current register values, .fp_state/.vr_state,
    or .transact_fp/.transact_vr) we should be using, since we have no
    way to tell if we are still in the same transaction, and if not,
    whether the previous transaction succeeded or failed.
    
    Thus it is necessary to strictly adhere to the rule that if FP has
    been enabled at any point in a transaction, we must keep FP enabled
    for the user process with the current transactional state in the
    FP registers, until we detect that it is no longer in a transaction.
    Similarly for VMX; once enabled it must stay enabled until the
    process is no longer transactional.
    
    In order to keep this rule, we add a new thread_info flag which we
    test when returning from the kernel to userspace, called TIF_RESTORE_TM.
    This flag indicates that there is FP/VMX/VSX state to be restored
    before entering userspace, and when it is set the .tm_orig_msr field
    in the thread_struct indicates what state needs to be restored.
    The restoration is done by restore_tm_state().  The TIF_RESTORE_TM
    bit is set by new giveup_fpu/altivec_maybe_transactional helpers,
    which are called from enable_kernel_fp/altivec, giveup_vsx, and
    flush_fp/altivec_to_thread instead of giveup_fpu/altivec.
    
    The other thing to be done is to get the transactional FP/VMX/VSX
    state from .fp_state/.vr_state when doing reclaim, if that state
    has been saved there by giveup_fpu/altivec_maybe_transactional.
    Having done this, we set the FP/VMX bit in the thread's MSR after
    reclaim to indicate that that part of the state is now valid
    (having been reclaimed from the processor's checkpointed state).
    
    Finally, in the signal handling code, we move the clearing of the
    transactional state bits in the thread's MSR a bit earlier, before
    calling flush_fp_to_thread(), so that we don't unnecessarily set
    the TIF_RESTORE_TM bit.
    
    This is the test program:
    
    /* Michael Neuling 4/12/2013
     *
     * See if the altivec state is leaked out of an aborted transaction due to
     * kernel vmx copy loops.
     *
     *   gcc -m64 htm_vmxcopy.c -o htm_vmxcopy
     *
     */
    
    /* We don't use all of these, but for reference: */
    
    int main(int argc, char *argv[])
    {
    	long double vecin = 1.3;
    	long double vecout;
    	unsigned long pgsize = getpagesize();
    	int i;
    	int fd;
    	int size = pgsize*16;
    	char tmpfile[] = "/tmp/page_faultXXXXXX";
    	char buf[pgsize];
    	char *a;
    	uint64_t aborted = 0;
    
    	fd = mkstemp(tmpfile);
    	assert(fd >= 0);
    
    	memset(buf, 0, pgsize);
    	for (i = 0; i < size; i += pgsize)
    		assert(write(fd, buf, pgsize) == pgsize);
    
    	unlink(tmpfile);
    
    	a = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
    	assert(a != MAP_FAILED);
    
    	asm __volatile__(
    		"lxvd2x 40,0,%[vecinptr] ; " // set 40 to initial value
    		TBEGIN
    		"beq	3f ;"
    		TSUSPEND
    		"xxlxor 40,40,40 ; " // set 40 to 0
    		"std	5, 0(%[map]) ;" // cause kernel vmx copy page
    		TABORT
    		TRESUME
    		TEND
    		"li	%[res], 0 ;"
    		"b	5f ;"
    		"3: ;" // Abort handler
    		"li	%[res], 1 ;"
    		"5: ;"
    		"stxvd2x 40,0,%[vecoutptr] ; "
    		: [res]"=r"(aborted)
    		: [vecinptr]"r"(&vecin),
    		  [vecoutptr]"r"(&vecout),
    		  [map]"r"(a)
    		: "memory", "r0", "r3", "r4", "r5", "r6", "r7");
    
    	if (aborted && (vecin != vecout)){
    		printf("FAILED: vector state leaked on abort %f != %f\n",
    		       (double)vecin, (double)vecout);
    		exit(1);
    	}
    
    	munmap(a, size);
    
    	close(fd);
    
    	printf("PASSED!\n");
    	return 0;
    }
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
    d31626f7
thread_info.h 6.14 KB