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Date: Thu, 22 Jan 2015 12:55:17 +0000
From: David Vrabel <david.vrabel@citrix.com>
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To: Andy Lutomirski <luto@amacapital.net>,
        "Luis R. Rodriguez" <mcgrof@do-not-panic.com>
CC: kvm list <kvm@vger.kernel.org>, "Luis R. Rodriguez" <mcgrof@suse.com>,
        X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        David Vrabel <david.vrabel@citrix.com>,
        Jan Beulich <JBeulich@suse.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to
 be preempted
References: <1421893039-426-1-git-send-email-mcgrof@do-not-panic.com>	<1421893039-426-3-git-send-email-mcgrof@do-not-panic.com> <CALCETrXSQ0JCKY5k0S_xX-i4+1BBVY+_9dv2ouj23WXFi8hyfg@mail.gmail.com>
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On 22/01/15 03:18, Andy Lutomirski wrote:
>> --- a/drivers/xen/events/events_base.c
>> +++ b/drivers/xen/events/events_base.c
>> @@ -32,6 +32,8 @@
>>  #include <linux/slab.h>
>>  #include <linux/irqnr.h>
>>  #include <linux/pci.h>
>> +#include <linux/sched.h>
>> +#include <linux/kprobes.h>
>>
>>  #ifdef CONFIG_X86
>>  #include <asm/desc.h>
>> @@ -1243,6 +1245,17 @@ void xen_evtchn_do_upcall(struct pt_regs *regs)
>>         set_irq_regs(old_regs);
>>  }
>>
>> +notrace void xen_end_upcall(struct pt_regs *regs)
>> +{
>> +       if (!xen_is_preemptible_hypercall(regs) ||
>> +           __this_cpu_read(xed_nesting_count))
>> +               return;
> 
> What's xed_nesting_count?

It used to prevent nested upcalls when a hypercall called from an upcall
triggers another upcall.

There's no way a such a nested hypercall can be preemptible so the check
cfor xed_nesting_count an be removed from here.

David