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 11:50:10 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	<david.vrabel@...rix.com>, <konrad.wilk@...cle.com>,
	<boris.ostrovsky@...cle.com>, <xen-devel@...ts.xenproject.org>
CC:	<kvm@...r.kernel.org>, "Luis R. Rodriguez" <mcgrof@...e.com>,
	<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 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.

~Andrew
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