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? 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
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