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[PATCH linux 3/8] x86/xen: use xen_vcpu_id mapping for HYPERVISOR_vcpu_op

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  [PATCH linux 3/8] x86/xen: use xen_vcpu_id mapping for HYPERVISOR_vcpu_op Vitaly Kuznetsov <vkuznets@redhat.com> - 2016-06-28 18:50 +0200
    Re: [Xen-devel] [PATCH linux 3/8] x86/xen: use xen_vcpu_id mapping  for HYPERVISOR_vcpu_op David Vrabel <david.vrabel@citrix.com> - 2016-06-29 14:30 +0200

#1433113 — [PATCH linux 3/8] x86/xen: use xen_vcpu_id mapping for HYPERVISOR_vcpu_op

HYPERVISOR_vcpu_op passes Linux's idea of vCPU id as a parameter while
Xen's idea is expected. In some cases these ideas diverge so we need to
do remapping.

There is an issue, however. PV guests do VCPUOP_is_up very early
(see xen_fill_possible_map() and xen_filter_cpu_maps()) when we don't have
perpu areas initialized. While it could be solved with switching to
early_percpu for xen_vcpu_id I think it's not worth it: PV guests will
probably never get to the point where their idea of vCPU id diverges from
Xen's.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/include/asm/xen/hypercall.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/xen/hypercall.h b/arch/x86/include/asm/xen/hypercall.h
index a12a047..20750c8 100644
--- a/arch/x86/include/asm/xen/hypercall.h
+++ b/arch/x86/include/asm/xen/hypercall.h
@@ -49,6 +49,7 @@
 #include <xen/interface/physdev.h>
 #include <xen/interface/platform.h>
 #include <xen/interface/xen-mca.h>
+#include <xen/xen-ops.h>
 
 /*
  * The hypercall asms have to meet several constraints:
@@ -422,7 +423,15 @@ HYPERVISOR_vm_assist(unsigned int cmd, unsigned int type)
 static inline int
 HYPERVISOR_vcpu_op(int cmd, int vcpuid, void *extra_args)
 {
-	return _hypercall3(int, vcpu_op, cmd, vcpuid, extra_args);
+	/*
+	 * PV guests call HYPERVISOR_vcpu_op before percpu areas are
+	 * initialized. As we always use direct mapping for vCPU ids
+	 * for them we can simply use Linux vcpuid here.
+	 */
+	return _hypercall3(int, vcpu_op, cmd,
+			   per_cpu(xen_vcpu_id, vcpuid) != -1 ?
+			   per_cpu(xen_vcpu_id, vcpuid) : vcpuid,
+			   extra_args);
 }
 
 #ifdef CONFIG_X86_64
-- 
2.5.5

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#1433704 — Re: [Xen-devel] [PATCH linux 3/8] x86/xen: use xen_vcpu_id mapping for HYPERVISOR_vcpu_op

On 28/06/16 17:47, Vitaly Kuznetsov wrote:
> HYPERVISOR_vcpu_op passes Linux's idea of vCPU id as a parameter while
> Xen's idea is expected. In some cases these ideas diverge so we need to
> do remapping.
> 
> There is an issue, however. PV guests do VCPUOP_is_up very early
> (see xen_fill_possible_map() and xen_filter_cpu_maps()) when we don't have
> perpu areas initialized. While it could be solved with switching to
> early_percpu for xen_vcpu_id I think it's not worth it: PV guests will
> probably never get to the point where their idea of vCPU id diverges from
> Xen's.
[...]
>  static inline int
>  HYPERVISOR_vcpu_op(int cmd, int vcpuid, void *extra_args)
>  {
> -	return _hypercall3(int, vcpu_op, cmd, vcpuid, extra_args);
> +	/*
> +	 * PV guests call HYPERVISOR_vcpu_op before percpu areas are
> +	 * initialized. As we always use direct mapping for vCPU ids
> +	 * for them we can simply use Linux vcpuid here.
> +	 */
> +	return _hypercall3(int, vcpu_op, cmd,
> +			   per_cpu(xen_vcpu_id, vcpuid) != -1 ?
> +			   per_cpu(xen_vcpu_id, vcpuid) : vcpuid,
> +			   extra_args);
>  }

HYPERVISOR_vcpu_op() should take Xen VCPUs, with the callers doing the
mapping.

David

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