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    Subject[PATCH v8 3/9] perf/amd/iommu: Misc fix up perf_iommu_read
    Date
    * Fix overflow handling since u64 delta would lose the MSB sign bit.
    * Remove unnecessary local64_cmpxchg().
    * Coding style and make use of GENMASK_ULL macro.

    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
    ---
    arch/x86/events/amd/iommu.c | 23 ++++++++++++-----------
    1 file changed, 12 insertions(+), 11 deletions(-)

    diff --git a/arch/x86/events/amd/iommu.c b/arch/x86/events/amd/iommu.c
    index 1aa25d8..3f1c18a 100644
    --- a/arch/x86/events/amd/iommu.c
    +++ b/arch/x86/events/amd/iommu.c
    @@ -320,9 +320,8 @@ static void perf_iommu_start(struct perf_event *event, int flags)

    static void perf_iommu_read(struct perf_event *event)
    {
    - u64 count = 0ULL;
    - u64 prev_raw_count = 0ULL;
    - u64 delta = 0ULL;
    + u64 count, prev;
    + s64 delta;
    struct hw_perf_event *hwc = &event->hw;

    amd_iommu_pc_get_set_reg_val(_GET_DEVID(event),
    @@ -330,18 +329,20 @@ static void perf_iommu_read(struct perf_event *event)
    IOMMU_PC_COUNTER_REG, &count, false);

    /* IOMMU pc counter register is only 48 bits */
    - count &= 0xFFFFFFFFFFFFULL;
    + count &= GENMASK_ULL(48, 0);

    - prev_raw_count = local64_read(&hwc->prev_count);
    - if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
    - count) != prev_raw_count)
    - return;
    + prev = local64_read(&hwc->prev_count);

    - /* Handling 48-bit counter overflowing */
    - delta = (count << COUNTER_SHIFT) - (prev_raw_count << COUNTER_SHIFT);
    + /*
    + * Since we do not enable counter overflow interrupts,
    + * we do not have to worry about prev_count changing on us.
    + */
    + local64_set(&hwc->prev_count, count);
    +
    + /* Handle 48-bit counter overflow */
    + delta = (count << COUNTER_SHIFT) - (prev << COUNTER_SHIFT);
    delta >>= COUNTER_SHIFT;
    local64_add(delta, &event->count);
    -
    }

    static void perf_iommu_stop(struct perf_event *event, int flags)
    --
    1.8.3.1
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