[Bug 100971] New: Doing a CLFLUSH on GTT-mapped memory causes a system reboot

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100971

            Bug ID: 100971
           Summary: Doing a CLFLUSH on GTT-mapped memory causes a system
                    reboot
           Product: DRI
           Version: XOrg git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: critical
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: jason at jlekstrand.net
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Hardware: BXT laptop

Kernel: 4.11.something.  I think it's one of Rob or Lionel's OA branches.  I'll
let Ken provide a few more details.

I was trying to figure out how to force the CPU to flush it's write-combine
buffers (the kernel wasn't flushing them when EXEC_OBJECT_ASYNC is set).  This
prompted me to start experimenting with different operations on GTT-mapped BOs
and see if I could do something to uncached memory that would trigger a WC
flush in the CPU from userspace.  I never succeeded, but I did manage to get
some interesting behavior.

The moment I added a CLFLUSH (which is an unprivileged command) to flush a
cache line that's mapped from the GTT, I immediately got system reboots.  It
seems 100% reproducable.  I've pushed a Vulkan driver branch here which
reproduces it:

https://cgit.freedesktop.org/~jekstrand/mesa/log/?h=bug/bxt-reboot

However, I think you could probably reproduce with an utterly trivial IGT test
if you don't want the full Vulkan driver.  I'm fairly sure the culprit is the
CLFLUSH of GTT-mapped memory.

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