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Subject: Re: [Xen-devel] [PATCH linux 3/8] x86/xen: use xen_vcpu_id mapping
 for HYPERVISOR_vcpu_op
To: Vitaly Kuznetsov <vkuznets@redhat.com>, <xen-devel@lists.xenproject.org>
References: <1467132449-1030-1-git-send-email-vkuznets@redhat.com>
 <1467132449-1030-4-git-send-email-vkuznets@redhat.com>
CC: Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>, <x86@kernel.org>,
        <linux-kernel@vger.kernel.org>, Julien Grall <julien.grall@citrix.com>,
        Ingo Molnar <mingo@redhat.com>,
        "David Vrabel" <david.vrabel@citrix.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>
From: David Vrabel <david.vrabel@citrix.com>
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Date: Wed, 29 Jun 2016 13:26:24 +0100
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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