On 2016-03-10 3:59 PM, Richard Purdie wrote:
On Wed, 2016-03-09 at 23:12 -0500, Bruce Ashfield wrote:
On 2016-03-09 4:23 PM, Richard Purdie wrote:
On Wed, 2016-03-09 at 13:53 -0500, Bruce Ashfield wrote:
As another follow up. The thread can be summarized as "It doesn't
look like it should have worked before, and qemu's pat emulation
may be the issue'.
The suggestion is to run with 'nopat', which is what Richard
originally
did.
So I'm going to prep a patch that drops the kernel patch, and
leaves
nopat enabled on the qemu command line. That should get us put
back
together in a semi-permanent way.
How sure are we this is a bug in QEMU's pat emulation? If that is
the
case we should file a bug against qemu and try and fix it rather
than
work around it...
It could still be something that the kernel can work around, Toshi
did say:
There is a matter of how qemu emulates CPU features. There is no
such
Intel CPU that supports PAT w/o MTRR. This is why the current code
assumes this dependency.
Which is likely the trigger, we've send information about the cpu to
him, and with that there's a chance for a pat fix.
He repeated our thought of running with 'nopat' while a fix is
considered.
It may be some time before that happens, and I was going to test
with the kernel patch dropped, and nopat in the qemu boot args. If
that works, I'd rather run with that, and then revisit when (if)
there's more changes upstream.
Reading the other thread, it looks like if MTRR is disabled, PAT needs
to be disabled too. That sounds like a simple enough patch which is
going upstream imminently so I think the preferred solution is to get
that into our kernels and then drop my patch?
Yep. Looks like there's a patch to disable the pat based on the flags
of the cpu model being proposed.
When that's out, I'll merge it, and then we'll drop this. So for now,
we wait a bit longer.
Bruce
Cheers,
Richard
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