linux-kernel - Re: [Xen-devel] [PATCH] x86, paravirt, xen: Remove the 64-bit irq_enable

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Date:	Mon, 06 Apr 2015 18:44:10 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Denys Vlasenko <dvlasenk@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	X86 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [Xen-devel] [PATCH] x86, paravirt, xen: Remove the 64-bit irq_enable_sysexit
 pvop

On 06/04/2015 16:29, Andy Lutomirski wrote:
> On Mon, Apr 6, 2015 at 7:10 AM, Konrad Rzeszutek Wilk
> <konrad.wilk@...cle.com> wrote:
>> On Fri, Apr 03, 2015 at 03:52:30PM -0700, Andy Lutomirski wrote:
>>> [cc: Boris and Konrad.  Whoops]
>>>
>>> On Fri, Apr 3, 2015 at 3:51 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>>> We don't use irq_enable_sysexit on 64-bit kernels any more.  Remove
>> Is there an commit (or name of patch) that explains why 32-bit-user-space-on-64-bit
>> kernels is unsavory?
> sysexit never tasted very good :-p
>
> We're (hopefully) not breaking 32-bit-user-space-on-64-bit, but we're
> trying an unconventional approach to making the code faster and less
> scary.  As a result, 64-bit kernels won't use sysexit any more.
> Hopefully Xen is okay with the slightly sneaky thing we're doing.
> AFAICT Xen thinks of sysretl and sysexit as slightly funny irets, so I
> don't expect there to be any problem.

64bit PV kernels must bounce through Xen to switch from the kernel to
the user pagetables (since both kernel and userspace are both actually
running in ring3 with user pages).

As a result, exit to userspace ends up as a hypercall into Xen which has
an effect very similar to an `iret`, but with some extra fixup in the
background.

I can't forsee any Xen issues as a result of this patch.

~Andrew
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