linux-kernel - Re: [Xen-devel] [PATCH linux 3/8] x86/xen: use xen_vcpu_id mapping for HYPERVISOR_vcpu

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Date:	Wed, 29 Jun 2016 13:26:24 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>,
	<xen-devel@...ts.xenproject.org>
CC:	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>,
	Julien Grall <julien.grall@...rix.com>,
	Ingo Molnar <mingo@...hat.com>,
	"David Vrabel" <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: 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