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Date: Fri, 23 Jan 2015 11:30:16 +0000
From: David Vrabel <david.vrabel@citrix.com>
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To: "Luis R. Rodriguez" <mcgrof@do-not-panic.com>, <david.vrabel@citrix.com>,
        <konrad.wilk@oracle.com>, <boris.ostrovsky@oracle.com>,
        <xen-devel@lists.xenproject.org>
CC: Borislav Petkov <bp@suse.de>, <kvm@vger.kernel.org>,
        "Luis R. Rodriguez" <mcgrof@suse.com>, <x86@kernel.org>,
        <linux-kernel@vger.kernel.org>, <rostedt@goodmis.org>,
        Andy Lutomirski <luto@amacapital.net>, Ingo Molnar <mingo@redhat.com>,
        Jan Beulich <JBeulich@suse.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Thomas Gleixner <tglx@linutronix.de>, <paulmck@linux.vnet.ibm.com>
Subject: Re: [Xen-devel] [RFC v4 1/2] x86/xen: add xen_is_preemptible_hypercall()
References: <1421972951-3940-1-git-send-email-mcgrof@do-not-panic.com> <1421972951-3940-2-git-send-email-mcgrof@do-not-panic.com>
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On 23/01/15 00:29, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@suse.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