linux-kernel - Re: [PATCH v4 00/17] x86: Rewrite exit-to-userspace code

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Date:	Mon, 29 Jun 2015 21:46:33 +0200
From:	Richard Weinberger <richard.weinberger@...il.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 00/17] x86: Rewrite exit-to-userspace code

Andy,

On Mon, Jun 29, 2015 at 9:33 PM, Andy Lutomirski <luto@...nel.org> wrote:
> This is the first big batch of x86 asm-to-C conversion patches.
>
> The exit-to-usermode code is copied in several places and is written
> in a nasty combination of asm and C.  It's not at all clear what
> it's supposed to do, and the way it's structured makes it very hard
> to work with.  For example, it's not even clear why syscall exit
> hooks are called only once per syscall right now.  (It seems to be a
> side effect of the way that rdi and rdx are handled in the asm loop,
> and it seems reliable, but it's still pointlessly complicated.)  The
> existing code also makes context tracking overly complicated and
> hard to understand.  Finally, it's nearly impossible for anyone to
> change what happens on exit to usermode, since the existing code is
> so fragile.
>
> I tried to clean it up incrementally, but I decided it was too hard.
> Instead, this series just replaces the code.  It seems to work.
>
> Context tracking in particular works very differently now.  The
> low-level entry code checks that we're in CONTEXT_USER and switches
> to CONTEXT_KERNEL.  The exit code does the reverse.  There is no
> need to track what CONTEXT_XYZ state we came from, because we
> already know.  Similarly, SCHEDULE_USER is gone, since we can
> reschedule if needed by simply calling schedule() from C code.
>
> The main things that are missing are that I haven't done the 32-bit
> parts (anyone want to help?) and therefore I haven't deleted the old
> C code.  I also think this may break UML for trivial reasons.
>
> IRQ context tracking is still messy.  One the cleanup progresses
> to the point that we can enter CONTEXT_KERNEL in syscalls before
> enabling interrupts, we can fully clean up IRQ context tracking.
>
> Once these land, I'll send some more :)
>
> Note: we might want to backport patches 1 and 2.
>
> Changes from v3:
>  - Add the syscall_arg_fault_32 test.
>  - Fix a pre-existing bad syscall arg buglet.
>  - Fix an asm glitch due to a bad rebase.
>  - Fix a CONFIG_PROVE_LOCKDEP warning.
> Borislav: the end result of this series differs from the v3.91 that I
> only in the removal of a single trailing tab.  The badarg patch is in
> a different place now, though, since we might want to backport it.
>
> Changes from v2: Misplaced the actual list -- sorry.
>
> Changes from v1:
>  - Fix bisection failure by squashing the 64-bit native and compat syscall
>    conversions together.  The intermediate state didn't built, and fixing
>    it isn't worthwhile (the results will be harder to understand).
>  - Replace context_tracking_assert_state with CT_WARN_ON and ct_state.
>  - The last two patches are now.  I incorrectly thought that we weren't
>    ready for them yet on 32-bit kernels, but I was wrong.
>
> Andy Lutomirski (16):
>   selftests/x86: Add a test for 32-bit fast syscall arg faults
>   x86/entry/64/compat: Fix bad fast syscall arg failure path
>   context_tracking: Add ct_state and CT_WARN_ON
>   notifiers: Assert that RCU is watching in notify_die
>   x86: Move C entry and exit code to arch/x86/entry/common.c
>   x86/traps: Assert that we're in CONTEXT_KERNEL in exception entries
>   x86/entry: Add enter_from_user_mode and use it in syscalls
>   x86/entry: Add new, comprehensible entry and exit hooks
>   x86/entry/64: Really create an error-entry-from-usermode code path
>   x86/entry/64: Migrate 64-bit and compat syscalls to new exit hooks
>   x86/asm/entry/64: Save all regs on interrupt entry
>   x86/asm/entry/64: Simplify irq stack pt_regs handling
>   x86/asm/entry/64: Migrate error and interrupt exit work to C
>   x86/entry: Remove exception_enter from most trap handlers
>   x86/entry: Remove SCHEDULE_USER and asm/context-tracking.h
>   x86/irq: Document how IRQ context tracking works and add an assertion
>
> Ingo Molnar (1):
>   uml: Fix do_signal() prototype

Do you have a git tree for that?

-- 
Thanks,
//richard
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