On Wed, Aug 20, 2014 at 11:18:21AM -0600, Adam Duskett wrote:
I have recently come upon this section of code in
arch/x86/kernel/cpu/mcheck/mce_amd.c that seems to be a redundant
unnecessary if check.
From line 170 - 176:
if (tr->set_lvt_off) {
if (lvt_off_valid(tr->b, tr->lvt_off, lo, hi)) {
/* set new lvt offset */
hi &= ~MASK_LVTOFF_HI;
hi |= tr->lvt_off << 20;
}
}
This seems like it's not actually doing anything because it's setting
the same value that the bit-field already has to itself.
I brought this up to Adam the other day, so he posted the question to this
list today to elicit a response from the original developer(s). I realize
the quickest response is to ask the original poster (Adam) to investigate
further, such as with pen and paper, but that is not a proper response to a
legitimate question. Here is the #define that is referenced, and the two
routines in question. This is current in kernel version 3.16 in
arch/x86/kernel/cpu/mcheck/mce_amd.c.
#define MASK_LVTOFF_HI 0x00F00000
static int lvt_off_valid(struct threshold_block *b, int apic, u32 lo, u32
hi)
{
int msr = (hi & MASK_LVTOFF_HI) >> 20;
if (apic < 0) {
pr_err(FW_BUG "cpu %d, failed to setup threshold interrupt "
"for bank %d, block %d (MSR%08X=0x%x%08x)\n", b->cpu,
b->bank, b->block, b->address, hi, lo);
return 0;
}
if (apic != msr) {
pr_err(FW_BUG "cpu %d, invalid threshold interrupt offset %d
"
"for bank %d, block %d (MSR%08X=0x%x%08x)\n",
b->cpu, apic, b->bank, b->block, b->address, hi, lo);
return 0;
}
return 1;
};
/*
* Called via smp_call_function_single(), must be called with correct
* cpu affinity.
*/
static void threshold_restart_bank(void *_tr)
{
struct thresh_restart *tr = _tr;
u32 hi, lo;
rdmsr(tr->b->address, lo, hi);
if (tr->b->threshold_limit < (hi & THRESHOLD_MAX))
tr->reset = 1; /* limit cannot be lower than err count */
if (tr->reset) { /* reset err count and overflow bit
*/
hi =
(hi & ~(MASK_ERR_COUNT_HI | MASK_OVERFLOW_HI)) |
(THRESHOLD_MAX - tr->b->threshold_limit);
} else if (tr->old_limit) { /* change limit w/o reset */
int new_count = (hi & THRESHOLD_MAX) +
(tr->old_limit - tr->b->threshold_limit);
hi = (hi & ~MASK_ERR_COUNT_HI) |
(new_count & THRESHOLD_MAX);
}
/* clear IntType */
hi &= ~MASK_INT_TYPE_HI;
if (!tr->b->interrupt_capable)
goto done;
if (tr->set_lvt_off) {
if (lvt_off_valid(tr->b, tr->lvt_off, lo, hi)) {
/* set new lvt offset */
hi &= ~MASK_LVTOFF_HI;
hi |= tr->lvt_off << 20;
}
}
if (tr->b->interrupt_enable)
hi |= INT_TYPE_APIC;
done:
hi |= MASK_COUNT_EN_HI;
wrmsr(tr->b->address, lo, hi);
}
If one were to actually analyze the source file from which this snippet
comes (lines 117 - 185), one would realize the call to lvt_off_valid() is
given tr->lvt_off as the input "apic" value that is compared to the content
in "hi" at bit positions 23:20 (MSR bits 55:52); this field is called LVT
Offset (LVTOFF). The value for tr->lvt_off is usually from 0 to 4,
inclusive. If this value is equal to the LVTOFF value in "hi", then
lvt_off_valid() returns 1 for true. If the value for tr->lvt_off differs
from the LVTOFF value in "hi", then lvt_off_valid() returns 0 for false.
Now, if the return from lvt_off_valid() is false, then nothing is changed in
"hi". However, if the return is true, which means the value in tr->lvt_off
is equal to the LVTOFF value in "hi", then the LVTOFF value in "hi" is
replaced with the value in tr->lvt_off. One has to wonder, then, why bother
actually calling lvt_off_valid() in the first place when the end result is
that "hi" does not change. What is the rationale for having the code
snippet at lines 170 - 176 when that condition check does nothing to change
"hi"?
--
Chip
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