linux-kernel - Re: [PATCH v5 2/2] x86/xen: allow privcmd hypercalls to be preempted on 64-bit

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Message-ID: <CAB=NE6XTv3hKOr=9gowTBry6Je=LOA+cJGu4wRawU-ur31z+4g@mail.gmail.com>
Date:	Mon, 2 Feb 2015 16:24:57 -0800
From:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Jan Beulich <JBeulich@...e.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
	kvm@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v5 2/2] x86/xen: allow privcmd hypercalls to be preempted
 on 64-bit

On Thu, Jan 29, 2015 at 12:35 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> On Tue, Jan 27, 2015 at 10:06:44AM +0000, David Vrabel wrote:
>> On 27/01/15 08:35, Jan Beulich wrote:
>> >>>> On 27.01.15 at 02:51, <mcgrof@...not-panic.com> wrote:
>> >
>> > Even if David told you this would be acceptable, I have to question
>> > an abstract model of fixing issues on only 64-bit kernels - this may
>> > be acceptable for distro purposes, but seems hardly the right
>> > approach for upstream. If 32-bit ones are to become deliberately
>> > broken, the XEN config option should become dependent on !X86_32.
>>
>> I'd rather have something omitted (keeping the current behaviour) than
>> something that has not been tested at all.
>>
>> Obviously it would be preferable to to fix both 32-bit and 64-bit x86
>> (and ARM as well) but I'm not going to block an important bug fix for
>> the majority use case (64-bit x86).
>
> Hey folks, what is the status of these patches? Any more feedback?

As I waited I decided to finally install a 32-bit OS but I found now
that as of Xen commit 5d1181a5 which went upstream as of xen 4.3 Xen
no longer supports 32-bit for x86. So -- Xen no loner even builds on
32-bit systems. The latest version I could use then would be Xen 4.2
but the even the stable-4.2 branch fails to build for me even after
fixing quite a bit of compile failures, I'm just going to give up now.
I want to build Xen as I'm relying on some customized emulated delays
on the hypervisor in order to test the issue, otherwise I'd have to
dedicate a huge system to reproduce this issue.

While it is possible folks on old xen may use newer kernels this issue
is likely not a high priority on those 32-bit systems and likely hard
to reproduce there. When and if someone is able to test this on a
32-bit kernel / hypervisor then they can test the small patch below.

Can we move forward with the 64-bit part then?

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 000d419..b4b1f42 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -982,6 +982,8 @@ ENTRY(xen_hypervisor_callback)
 ENTRY(xen_do_upcall)
 1:    mov %esp, %eax
     call xen_evtchn_do_upcall
+    movl %esp,%eax
+    call xen_end_upcall
     jmp  ret_from_intr
     CFI_ENDPROC
 ENDPROC(xen_hypervisor_callback)
--
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