linux-kernel - Re: [PATCH 4/8] x86/vm86: Use the normal pt

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Date:	Wed, 29 Jul 2015 10:50:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Brian Gerst <brgerst@...il.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH 4/8] x86/vm86: Use the normal pt_regs area for vm86

On Wed, Jul 29, 2015 at 10:47 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Jul 29, 2015 at 10:36 AM, Brian Gerst <brgerst@...il.com> wrote:
>>
>> I think I remember now what the issue was.  Since entering vm86 mode
>> uses force_iret(), the work_pending code path was being taken.  I had
>> to move the call to save_v86_state out of here to handle_signal(),
>> otherwise it would just restore the 32-bit regs and exit the syscall
>> without ever entering vm86 mode.  But that meant that the test for
>> kernel CS was seeing the vm86 regs instead of 32-bit regs, and was
>> failing because it didn't account for the VM bit (if the real-mode CS
>> looked like RPL 0).  A fault would get stuck in a loop because it
>> couldn't exit to the signal handling code.
>
> Hmm. I think we'd want a big comment about that code then, and how it
> is only used for user-space (including vm86 mode).

I think that a very tiny comment might be sufficient, because I'm
planning to resubmit my patch series that removes all the work_xyz asm
entirely as soon as Brian's patches land in -tip :)  I've just been
waiting for v86 mode to GTFO of the asm loop so the rest could be done
in C.  I had a small patch to deal with vm86, but Brian's is much
better, so I decided to hold off.

--Andy
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