On 03/19/2015 02:51 AM, James Hogan wrote:
On 18/03/15 23:25, Leonid Yegoshin wrote:
On 03/18/2015 03:12 PM, James Hogan wrote:
Hi Leonid,
On Wed, Mar 18, 2015 at 12:46:51PM -0700, Leonid Yegoshin wrote:
thread_msa_context_live() == check of TIF_MSA_CTX_LIVE == existence of
MSA context for thread.
It differs from MSA is owned by thread, it just says that thread has
already initialized MSA.
Unfortunate choice of function name, I believe.
Right (I mis-read when its cleared when i grepped). Still, that would
make it even harder to hit since lose_fpu wouldn't clear it, and you
already would've taken an MSA disabled exception first.
No, lose_fpu disables MSA now, saves MSA context and switches off
TIF_USEDMSA. See 33c771ba5c5d067f85a5a6c4b11047219b5b8f4e, "MIPS:
save/disable MSA in lose_fpu".
However, a process still has MSA context initialized and it is indicated
by TIF_MSA_CTX_LIVE.
It should have it before it can get any AdE exception on MSA instruction.
Yes, exactly.
Anyway, my point was that there's nothing invalid about an unaligned
load being the first MSA instruction. You might use it to load the
initial vector state.
No, it is invalid. If MSA is disabled it should trigger "MSA Disabled"
exception.
It's valid for the user to start their program with a ld.b.
As you say, it'll raise an MSA disabled exception first though. The
handler will own MSA, and set TIF_MSA_CTX_LIVE, which makes the check
pointless?
"Unfortunately, some HW versions had AdE first and it may be logical
from some HW point (if access is done before instruction is completely
decoded). But that is wrong."
I suppose an AdE from a normal unaligned load could still race with
another thread modifying the instruction to an MSA ld.b, but even if it
did, I don't think it would do any harm?
Unfortunately, some HW versions had AdE first and it may be logical from
some HW point (if access is done before instruction is completely
decoded). But that is wrong.
Yes, MSA Disabled would clearly come under "Instruction Validity
Exceptions", which is very sensibly higher priority than "Address error
- Data access".
Anyway, at the very least it needs a comment to justify what it is
trying to catch and what harm it is trying to avoid, since it isn't
obvious, and tbh seems pointless.
Cheers
James
