Re: [PATCH v2 1/2] x86/xen: remove xen_have_vcpu_info_placement flag

[Juergen Gross]

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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