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

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Date:	Mon, 06 Apr 2015 14:30:20 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	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>,
	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 04/06/2015 01:44 PM, Andrew Cooper wrote:
> 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.


I ran tip plus this patch (plus another patch that fixes an unrelated 
Xen regression in tip) through our test suite and it completed without 
problems.

I also ran some very simple 32-bit programs in a 64-bit PV guest and 
didn't see any problems there neither.

-boris

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