linux-kernel - Re: [PATCH 3/5] xen/smp: Update pv_lock_ops functions before alternative code starts under PVHVM

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Date:	Mon, 9 Sep 2013 11:31:48 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
CC:	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>, <stefan.bader@...onical.com>,
	<stefano.stabellini@...citrix.com>, <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 3/5] xen/smp: Update pv_lock_ops functions before alternative
 code starts under PVHVM

On 07/09/13 14:46, Konrad Rzeszutek Wilk wrote:
> Before this patch we would patch all of the pv_lock_ops sites
> using alternative assembler. Then later in the bootup cycle
> change the unlock_kick and lock_spinning to the Xen specific -
> without re patching.
> 
> That meant that for the core of the kernel we would be running
> with the baremetal version of unlock_kick and lock_spinning while
> for modules we would have the proper Xen specific slowpaths.
> 
> As most of the module uses some API from the core kernel that ended
> up with slowpath lockers waiting forever to be kicked (b/c they
> would be using the Xen specific slowpath logic). And the
> kick never came b/c the unlock path that was taken was the
> baremetal one.
> 
> On PV we do not have the problem as we initialise before the
> alternative code kicks in.
> 
> The fix is to move the updating of the pv_lock_ops function
> before the alternative code starts patching.

This comment seems odd.  The xen_spinlock_init() call is added not moved.

> --- a/arch/x86/xen/smp.c
> +++ b/arch/x86/xen/smp.c
> @@ -731,4 +731,12 @@ void __init xen_hvm_smp_init(void)
>  	smp_ops.cpu_die = xen_hvm_cpu_die;
>  	smp_ops.send_call_func_ipi = xen_smp_send_call_function_ipi;
>  	smp_ops.send_call_func_single_ipi = xen_smp_send_call_function_single_ipi;
> +
> +	/*
> +	 * The alternative logic (which patches the unlock/lock) runs before
> +	 * the smp bootup up code is activated. That meant we would never patch
> +	 * the core of the kernel with proper paravirt interfaces but would patch
> +	 * modules.
> +	 */
> +	xen_init_spinlocks();

PV does this in xen_smp_prepare_boot_cpu.  It would be better if the
PVHVM case followed this same pattern and provide a smp_prepare_boot_cpu
implementation to do this?

David
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