linux-kernel - Re: [PATCH v2 1/2] x86/xen: remove xen_have_vcpu_info

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Date:   Wed, 22 Sep 2021 17:43:41 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 1/2] x86/xen: remove xen_have_vcpu_info_placement flag


On 9/22/21 6:31 AM, Juergen Gross wrote:
>  
> -	if (xen_have_vcpu_info_placement) {
> -		vcpup = &per_cpu(xen_vcpu_info, cpu);
> -		info.mfn = arbitrary_virt_to_mfn(vcpup);
> -		info.offset = offset_in_page(vcpup);
> +	vcpup = &per_cpu(xen_vcpu_info, cpu);
> +	info.mfn = arbitrary_virt_to_mfn(vcpup);
> +	info.offset = offset_in_page(vcpup);
>  
> -		/*
> -		 * Check to see if the hypervisor will put the vcpu_info
> -		 * structure where we want it, which allows direct access via
> -		 * a percpu-variable.
> -		 * N.B. This hypercall can _only_ be called once per CPU.
> -		 * Subsequent calls will error out with -EINVAL. This is due to
> -		 * the fact that hypervisor has no unregister variant and this
> -		 * hypercall does not allow to over-write info.mfn and
> -		 * info.offset.
> -		 */
> -		err = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info,
> -					 xen_vcpu_nr(cpu), &info);
> -
> -		if (err) {
> -			pr_warn_once("register_vcpu_info failed: cpu=%d err=%d\n",
> -				     cpu, err);
> -			xen_have_vcpu_info_placement = 0;
> -		} else {
> -			/*
> -			 * This cpu is using the registered vcpu info, even if
> -			 * later ones fail to.
> -			 */
> -			per_cpu(xen_vcpu, cpu) = vcpup;
> -		}
> -	}
> -
> -	if (!xen_have_vcpu_info_placement)
> -		xen_vcpu_info_reset(cpu);
> +	/*
> +	 * N.B. This hypercall can _only_ be called once per CPU.
> +	 * Subsequent calls will error out with -EINVAL. This is due to
> +	 * the fact that hypervisor has no unregister variant and this
> +	 * hypercall does not allow to over-write info.mfn and
> +	 * info.offset.
> +	 */
> +	err = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, xen_vcpu_nr(cpu),
> +				 &info);
> +	if (err)
> +		panic("register_vcpu_info failed: cpu=%d err=%d\n", cpu, err);
>  


This is change in behavior. Before if the hypercall failed we still try to boot. I am not sure we need to worry about this (since it's not clear it actually works)  but I'd at least mention this in the commit message.


-boris