On 7/12/19 3:51 PM, Dave Hansen wrote:
On 7/12/19 1:09 AM, Alexandre Chartre wrote:
On 7/12/19 12:38 AM, Dave Hansen wrote:
I don't see the per-cpu areas in here. But, the ASI macros in
entry_64.S (and asi_start_abort()) use per-cpu data.
We don't map all per-cpu areas, but only the per-cpu variables we need. ASI
code uses the per-cpu cpu_asi_session variable which is mapped when an ASI
is created (see patch 15/26):
No fair! I had per-cpu variables just for PTI at some point and had to
give them up! ;)
+ /*
+ * Map the percpu ASI sessions. This is used by interrupt handlers
+ * to figure out if we have entered isolation and switch back to
+ * the kernel address space.
+ */
+ err = ASI_MAP_CPUVAR(asi, cpu_asi_session);
+ if (err)
+ return err;
Also, this stuff seems to do naughty stuff (calling C code, touching
per-cpu data) before the PTI CR3 writes have been done. But, I don't
see anything excluding PTI and this code from coexisting.
My understanding is that PTI CR3 writes only happens when switching to/from
userland. While ASI enter/exit/abort happens while we are already in the
kernel,
so asi_start_abort() is not called when coming from userland and so not
interacting with PTI.
OK, that makes sense. You only need to call C code when interrupted
from something in the kernel (deeper than the entry code), and those
were already running kernel C code anyway.
Exactly.
If this continues to live in the entry code, I think you have a good
clue where to start commenting.
Yeah, lot of writing to do... :-)
BTW, the PTI CR3 writes are not *strictly* about the interrupt coming
from user vs. kernel. It's tricky because there's a window both in the
entry and exit code where you are in the kernel but have a userspace CR3
value. You end up needing a CR3 write when you have a userspace CR3
value when the interrupt occurred, not only when you interrupt userspace
itself.
Right. ASI is simpler because it comes from the kernel and return to the
kernel. There's just a small window (on entry) where we have the ASI CR3
but we quickly switch to the full kernel CR3.
alex.
