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

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Date:   Thu, 23 Sep 2021 06:44:50 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.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 22.09.21 23:43, Boris Ostrovsky wrote:
> 
> 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.

Hmm, maybe I should have been more explicit saying that the hypercall
was introduced in Xen 3.4, and only reason of failure is either an
illegal vcpu, an invalid mapping specification, or a try to reissue the
hypercall for a vcpu. None of those should ever happen.


Juergen

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