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Commit fdfd811d authored by David Vrabel's avatar David Vrabel
Browse files

x86/xen: allow privcmd hypercalls to be preempted 



Hypercalls submitted by user space tools via the privcmd driver can
take a long time (potentially many 10s of seconds) if the hypercall
has many sub-operations.

A fully preemptible kernel may deschedule such as task in any upcall
called from a hypercall continuation.

However, in a kernel with voluntary or no preemption, hypercall
continuations in Xen allow event handlers to be run but the task
issuing the hypercall will not be descheduled until the hypercall is
complete and the ioctl returns to user space.  These long running
tasks may also trigger the kernel's soft lockup detection.

Add xen_preemptible_hcall_begin() and xen_preemptible_hcall_end() to
bracket hypercalls that may be preempted.  Use these in the privcmd
driver.

When returning from an upcall, call xen_maybe_preempt_hcall() which
adds a schedule point if if the current task was within a preemptible
hypercall.

Since _cond_resched() can move the task to a different CPU, clear and
set xen_in_preemptible_hcall around the call.

Signed-off-by: default avatarDavid Vrabel <david.vrabel@citrix.com>
Reviewed-by: default avatarBoris Ostrovsky <boris.ostrovsky@oracle.com>
parent 31795b47

arch/x86/kernel/entry_32.S

+3 −0
Original line number Diff line number Diff line
@@ -982,6 +982,9 @@ ENTRY(xen_hypervisor_callback) 
ENTRY(xen_do_upcall)

1:	mov %esp, %eax

	call xen_evtchn_do_upcall

#ifndef CONFIG_PREEMPT

	call xen_maybe_preempt_hcall

#endif

	jmp  ret_from_intr

	CFI_ENDPROC

ENDPROC(xen_hypervisor_callback)

arch/x86/kernel/entry_64.S

+3 −0
Original line number Diff line number Diff line
@@ -1208,6 +1208,9 @@ ENTRY(xen_do_hypervisor_callback) # do_hypervisor_callback(struct *pt_regs) 
	popq %rsp

	CFI_DEF_CFA_REGISTER rsp

	decl PER_CPU_VAR(irq_count)

#ifndef CONFIG_PREEMPT

	call xen_maybe_preempt_hcall

#endif

	jmp  error_exit

	CFI_ENDPROC

END(xen_do_hypervisor_callback)

drivers/xen/Makefile

+1 −1
Original line number Diff line number Diff line
@@ -2,7 +2,7 @@ ifeq ($(filter y, $(CONFIG_ARM) $(CONFIG_ARM64)),) 
obj-$(CONFIG_HOTPLUG_CPU)		+= cpu_hotplug.o

endif

obj-$(CONFIG_X86)			+= fallback.o

obj-y	+= grant-table.o features.o balloon.o manage.o

obj-y	+= grant-table.o features.o balloon.o manage.o preempt.o

obj-y	+= events/

obj-y	+= xenbus/

drivers/xen/preempt.c

0 → 100644
+44 −0
Original line number Diff line number Diff line 
/*

 * Preemptible hypercalls

 *

 * Copyright (C) 2014 Citrix Systems R&D ltd.

 *

 * This source code is free software; you can redistribute it and/or

 * modify it under the terms of the GNU General Public License as

 * published by the Free Software Foundation; either version 2 of the

 * License, or (at your option) any later version.

 */



#include <linux/sched.h>

#include <xen/xen-ops.h>



#ifndef CONFIG_PREEMPT



/*

 * Some hypercalls issued by the toolstack can take many 10s of

 * seconds. Allow tasks running hypercalls via the privcmd driver to

 * be voluntarily preempted even if full kernel preemption is

 * disabled.

 *

 * Such preemptible hypercalls are bracketed by

 * xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()

 * calls.

 */



DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);

EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);



asmlinkage __visible void xen_maybe_preempt_hcall(void)

{

	if (unlikely(__this_cpu_read(xen_in_preemptible_hcall)

		     && should_resched())) {

		/*

		 * Clear flag as we may be rescheduled on a different

		 * cpu.

		 */

		__this_cpu_write(xen_in_preemptible_hcall, false);

		_cond_resched();

		__this_cpu_write(xen_in_preemptible_hcall, true);

	}

}

#endif /* CONFIG_PREEMPT */

drivers/xen/privcmd.c

+2 −0
Original line number Diff line number Diff line
@@ -56,10 +56,12 @@ static long privcmd_ioctl_hypercall(void __user *udata) 
	if (copy_from_user(&hypercall, udata, sizeof(hypercall)))

		return -EFAULT;



	xen_preemptible_hcall_begin();

	ret = privcmd_call(hypercall.op,

			   hypercall.arg[0], hypercall.arg[1],

			   hypercall.arg[2], hypercall.arg[3],

			   hypercall.arg[4]);

	xen_preemptible_hcall_end();



	return ret;

}