From: Finn Thain <fthain@linux-m68k.org>
To: Michael Schmitz <schmitzmic@gmail.com>
Cc: geert@linux-m68k.org, linux-m68k@lists.linux-m68k.org
Subject: Re: [PATCH RFC v2 1/2] m68k: Handle __generic_copy_to_user faults more carefully
Date: Thu, 25 Apr 2024 16:32:35 +1000 (AEST) [thread overview]
Message-ID: <36a70661-3bfc-46f1-f211-fa87f883371a@linux-m68k.org> (raw)
In-Reply-To: <5d1a552e-f66d-dbbb-5630-42d22870dd15@gmail.com>
On Thu, 25 Apr 2024, Michael Schmitz wrote:
> >
> > I've just tested this on a Motorola 68040 and I got an oops in
> > __generic_copy_to_user(). It appears that we need more entries in the
> > exception table. (I also tested in QEMU and it did not oops.)
>
> I'm a bit puzzled about the location of the fault.
>
> The values of faddr and a0 from the exception frame indicate that the
> transfer leading up to the fault was a longword transfer. Both ssw and
> wbs2 suggest the same. Yet we don't take the fault on the longword
> moves, but apparently on the word moves right after.
>
> That can't be right either - d1 is 1 so the word moves would have been
> skipped. It appears that we only take the movesl fault the next time any
> bus cycle is initiated on 040 (the moveb at 0x46454c).
>
Seems so.
> That's different from how the 030 faulted in the same situation. I
> expect we'll have to add exception table entries on the movew and moveb
> instructions, too. I'll do that next.
>
> > This oops indicates that we are going to need the final NOP that was
> > in the first version of your patch. My test program seems inadequate
> > to show that it is safe to omit that NOP -- we would need a test which
> > doesn't jump over the MOVES.B.
>
> We'd need a test using any number of longword moves expected to succeed,
> followed by a byte move which is expected to fault. The current test
> would attempt to do a byte move, but faults during the longword moves.
>
> This requires running the test program in a directory whose absolute
> path is a multiple of four characters long, and setting the start
> address for the getcwd test accordingly, so the newline at the end of
> the string is the single byte left to copy. Does that make sense?
>
Yes (I take it you meant NUL instead of LF). But my concern was that the
test program passes a pointer like 0xc0029000 - 1. That means the final
byte will land on a word that already faulted. I'll need to add a new test
that passes a pointer like 0xc0029000 - 5.
> Incidentally - what is the path this tests is run in? Any path longer
> than five characters (including the newline) would have to had looped
> back to the first movel, and faulted there?
>
It was /tmp.
> As you said before - we'd need to know a lot more about
> microarchitectural details here.
>
It's hard to be certain. We just have to experiment until we find
something that works on the CPUs we can test.
next prev parent reply other threads:[~2024-04-25 6:32 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-22 2:29 [PATCH RFC v2 0/2] m68k uaccess fault handling fixes Michael Schmitz
2024-04-22 2:29 ` [PATCH RFC v2 1/2] m68k: Handle __generic_copy_to_user faults more carefully Michael Schmitz
2024-04-25 4:16 ` Finn Thain
2024-04-25 5:32 ` Michael Schmitz
2024-04-25 6:32 ` Finn Thain [this message]
2024-04-25 7:52 ` Michael Schmitz
2024-04-25 5:45 ` Michael Schmitz
2024-04-25 6:47 ` Finn Thain
2024-04-25 7:43 ` Michael Schmitz
2024-04-25 8:20 ` Michael Schmitz
2024-04-25 19:15 ` Michael Schmitz
2024-04-26 1:00 ` Finn Thain
2024-04-26 1:22 ` Michael Schmitz
2024-04-26 7:10 ` Michael Schmitz
2024-04-26 7:57 ` Finn Thain
2024-04-26 8:31 ` Michael Schmitz
2024-04-26 7:58 ` Finn Thain
2024-04-27 1:44 ` Finn Thain
2024-04-27 4:41 ` Michael Schmitz
2024-04-22 2:29 ` [PATCH RFC v2 2/2] m68k: improve __constant_copy_to_user_asm() fault handling Michael Schmitz
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