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

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Message-ID: <20150122184100.GP17887@wotan.suse.de>
Date:	Thu, 22 Jan 2015 19:41:00 +0100
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	david.vrabel@...rix.com, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, xen-devel@...ts.xenproject.org,
	kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...e.de>
Subject: Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to
	be preempted

On Thu, Jan 22, 2015 at 11:50:10AM +0000, Andrew Cooper wrote:
> On 22/01/15 02:17, Luis R. Rodriguez 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;
> > +
> > +	if (_cond_resched())
> > +		printk(KERN_DEBUG "xen hypercall preempted\n");
> 
> I wouldn't even put this at debug level.  On a large server with plenty
> of domains being created/migrated/destroyed, it is quite likely that a
> toolstack task might get preempted in this way.
> 
> I don't believe the message is of any practical use.

I'll just nuke it then.

 Luis
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