linux-kernel - Re: [Xen-devel] [RFC v4 1/2] x86/xen: add xen_is_preemptible

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Message-ID: <54C230C8.4000207@citrix.com>
Date:	Fri, 23 Jan 2015 11:30:16 +0000
From:	David Vrabel <david.vrabel@...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:	Borislav Petkov <bp@...e.de>, <kvm@...r.kernel.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, <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>,
	<paulmck@...ux.vnet.ibm.com>
Subject: Re: [Xen-devel] [RFC v4 1/2] x86/xen: add xen_is_preemptible_hypercall()

On 23/01/15 00:29, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@...e.com>
> 
> On kernels with voluntary or no preemption we can run
> into situations where a hypercall issued through userspace
> will linger around as it addresses sub-operatiosn in kernel
> context (multicalls). Such operations can trigger soft lockup
> detection.
> 
> We want to address a way to let the kernel voluntarily preempt
> such calls even on non preempt kernels, to address this we first
> need to distinguish which hypercalls fall under this category.
> This implements xen_is_preemptible_hypercall() which lets us do
> just that by adding a secondary hypercall page, calls made via
> the new page may be preempted.
[...]
> --- a/arch/x86/include/asm/xen/hypercall.h
> +++ b/arch/x86/include/asm/xen/hypercall.h
> @@ -84,6 +84,22 @@
>  
>  extern struct { char _entry[32]; } hypercall_page[];
>  
> +#ifndef CONFIG_PREEMPT
> +extern struct { char _entry[32]; } preemptible_hypercall_page[];
> +
> +static inline bool xen_is_preemptible_hypercall(struct pt_regs *regs)
> +{
> +	return !user_mode_vm(regs) &&
> +		regs->ip >= (unsigned long)preemptible_hypercall_page &&
> +		regs->ip < (unsigned long)preemptible_hypercall_page + PAGE_SIZE;

I think you can optimize this to:

    return (regs->ip >> PAGE_SHIFT) == preemptible_hypercall_pfn
	&& !user_mode_vm(regs);

David

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