linux-kernel - Re: [PATCH] Make math_state_restore() save and restore the interrupt flag

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Date:	Mon, 03 Feb 2014 22:03:26 -0800
From:	Suresh Siddha <sbsiddha@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nate Eldredge <nate@...tsmathematics.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	stable <stable@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Maarten Baert <maarten-baert@...mail.com>,
	Jan Kara <jack@...e.cz>, George Spelvin <linux@...izon.com>,
	Pekka Riikonen <priikone@....fi>
Subject: Re: [PATCH] Make math_state_restore() save and restore the
 interrupt flag

On Mon, 2014-02-03 at 10:20 -0800, Linus Torvalds wrote:
> Thinking about it some more, this patch is *almost* not needed at all.
> 
> I'm wondering if you should just change the first patch to just always
> initialize the fpu when it is allocated, and at execve() time (ie in
> flush_thread()).
> 

We already do this for eager-fpu case, in eager_fpu_init() during boot
and in drop_init_fpu() during flush_thread().

> If we do that, then this:
> 
> +       if (!tsk_used_math(tsk))
> +               init_fpu(tsk);
> 
> can be dropped entirely from math_state_restore(). 

yeah, probably for eager-fpu, but:

> And quite frankly,
> at that point, I think all the changes to __kernel_fpu_end() can go
> away, because at that point math_state_restore() really does the right
> thing - all the allocations are gone, and all the async task state
> games are gone, only the "restore state" remains.
> 
> Hmm? So the only thing needed would be to add that "init_fpu()" to the
> initial bootmem allocation path and to change flush_thread() (it
> currently does "drop_init_fpu()", let's just make it initialize the
> FPU state using fpu_finit()), and then we could remove the whole
> "used_math" bit entirely, and just say that the FPU is always
> initialized.
> 
> What do you guys think?

No. as I mentioned in the changelog, there is one more path which does
drop_fpu() and we still depend on this used_math bit for eager-fpu.

in signal restore path for 32-bit app, where we copy the sig-context
state from the user stack to the kernel manually (because of legacy
reasons where fsave state is followed by fxsave state etc in the 32-bit
signal handler context and we have to go through convert_to_fxsr() etc).

from __restore_xstate_sig() :

                /*
                 * Drop the current fpu which clears used_math(). This ensures
                 * that any context-switch during the copy of the new state,
                 * avoids the intermediate state from getting restored/saved.
                 * Thus avoiding the new restored state from getting corrupted.
                 * We will be ready to restore/save the state only after
                 * set_used_math() is again set.
                 */
                drop_fpu(tsk);


thanks,
suresh

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