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From: David Laight <[email protected]>
To: 'Linus Torvalds' <[email protected]>,
	Stefan Metzmacher <[email protected]>
Cc: Thomas Gleixner <[email protected]>,
	Andy Lutomirski <[email protected]>, Jens Axboe <[email protected]>,
	Linux Kernel Mailing List <[email protected]>,
	io-uring <[email protected]>,
	"the arch/x86 maintainers" <[email protected]>
Subject: RE: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads
Date: Tue, 4 May 2021 08:22:05 +0000	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAHk-=wiR7c-UHh_3Rj-EU8=AbURKchnMFJWW7=5EH=qEUDT8wg@mail.gmail.com>

From: Linus Torvalds
> Sent: 04 May 2021 00:48
> 
> On Mon, May 3, 2021 at 4:27 PM Stefan Metzmacher <[email protected]> wrote:
> >
> > If I remember correctly gdb showed bogus addresses for the backtraces of the io_threads,
> > as some regs where not cleared.
> 
> Yeah, so that patch will make the IO thread have the user stack
> pointer point to the original user stack, but that stack will
> obviously be used by the original thread which means that it will
> contain random stuff on it.
> 
> Doing a
> 
>         childregs->sp = 0;
> 
> is probably a good idea for that PF_IO_WORKER case, since it really
> doesn't have - or need - a user stack.
> 
> Of course, it doesn't really have - or need - any of the other user
> registers either, but once you fill in the segment stuff to make gdb
> happy, you might as well fill it all in using the same code that the
> regular case does.

Presumably gdb can only read/write the 'user' registers (normally
saved on kernel entry).
Since these will never be loaded it really doesn't matter (to the
kernel) what is returned to gdb or what gdb writes into them.
The same ought to be true of the FP state.

If gdb writes to an FP (etc) register the process doesn't currently
have (eg an AVX512 register) then that is not really different from
doing it to a normal process.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  parent reply	other threads:[~2021-05-04  8:22 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <[email protected]>
2021-05-03 16:05 ` [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads Andy Lutomirski
2021-05-03 19:14   ` Linus Torvalds
2021-05-03 20:15     ` Andy Lutomirski
2021-05-03 20:21       ` Andy Lutomirski
2021-05-03 20:37       ` Linus Torvalds
2021-05-03 21:26         ` Jens Axboe
2021-05-03 21:49           ` Andy Lutomirski
2021-05-03 22:08             ` Linus Torvalds
2021-05-03 22:56               ` Thomas Gleixner
2021-05-03 23:15                 ` Andy Lutomirski
2021-05-03 23:16                 ` Linus Torvalds
2021-05-03 23:19                   ` Linus Torvalds
2021-05-03 23:27                   ` Stefan Metzmacher
2021-05-03 23:48                     ` Linus Torvalds
2021-05-04  2:50                       ` Jens Axboe
2021-05-04 11:39                         ` Stefan Metzmacher
2021-05-04 15:53                           ` Linus Torvalds
2021-05-12  4:24                         ` Olivier Langlois
2021-05-12 17:44                           ` Linus Torvalds
2021-05-12 20:55                             ` Jens Axboe
2021-05-20  4:13                               ` Olivier Langlois
2021-05-21  7:31                                 ` Olivier Langlois
2021-05-25 19:39                                   ` Olivier Langlois
2021-05-25 19:45                                     ` Olivier Langlois
2021-05-25 19:52                                     ` Jens Axboe
2021-05-25 20:23                                     ` Linus Torvalds
2021-05-04  8:22                       ` David Laight [this message]
2021-05-04  0:01                   ` Andy Lutomirski
2021-05-04  8:39     ` Peter Zijlstra
2021-05-04 15:35       ` Borislav Petkov
2021-05-04 15:55         ` Simon Marchi
2021-05-05 11:29           ` Stefan Metzmacher
2021-05-05 21:59             ` Simon Marchi
2021-05-05 22:11               ` Andy Lutomirski
2021-05-05 23:12                 ` Borislav Petkov
2021-05-05 23:22                   ` Andy Lutomirski
2021-05-06  1:04                 ` Simon Marchi
2021-05-06 15:11                   ` Andy Lutomirski
2021-05-06  9:47                 ` David Laight
2021-05-06  9:53                   ` David Laight
2021-05-05 22:21               ` Stefan Metzmacher
2021-05-05 23:15                 ` Simon Marchi
2021-04-11 15:27 Stefan Metzmacher
2021-04-14 21:28 ` Stefan Metzmacher

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