linux-kernel - Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be preempted

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Date:	Thu, 22 Jan 2015 12:55:17 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
CC:	kvm list <kvm@...r.kernel.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to
 be preempted

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
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