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

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Message-ID: <CAMzpN2h94kXFnS9Shdhb=b9k95ar7omC06jrWky2P4tO0dZdpQ@mail.gmail.com>
Date:	Wed, 29 Jul 2015 13:36:52 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>, 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 1:16 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Jul 29, 2015 at 10:14 AM, Brian Gerst <brgerst@...il.com> wrote:
>>
>> I think it was causing signal handling to fail, but I can't remember
>> exactly.
>
> Ugh.
>
> If that hunk made a difference, then there is something wrong with
> your patch-series. So please double-check.
>
>                 Linus

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.

--
Brian Gerst
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