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SubjectRe: [PATCH v2 30/45] KVM: Move vcpu alloc and init invocation to common code
On Fri, Dec 20, 2019 at 10:33:25AM +0100, Cornelia Huck wrote:
> On Wed, 18 Dec 2019 13:55:15 -0800
> Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> > +int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> > {
> > - struct kvm_vcpu *vcpu;
> > struct sie_page *sie_page;
> > int rc;
> >
> > - rc = -ENOMEM;
> > -
> > - vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL);
> > - if (!vcpu)
> > - goto out;
> > -
> > - rc = kvm_vcpu_init(vcpu, kvm, id);
> > - if (rc)
> > - goto out_free_cpu;
> > -
> > - rc = -ENOMEM;
> > -
> > BUILD_BUG_ON(sizeof(struct sie_page) != 4096);
> > sie_page = (struct sie_page *) get_zeroed_page(GFP_KERNEL);
> > if (!sie_page)
> > - goto out_uninit_vcpu;
> > + return -ENOMEM;
> >
> > vcpu->arch.sie_block = &sie_page->sie_block;
> > vcpu->arch.sie_block->itdba = (unsigned long) &sie_page->itdb;
> > @@ -3087,15 +3070,11 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
> > vcpu->arch.sie_block);
> > trace_kvm_s390_create_vcpu(id, vcpu, vcpu->arch.sie_block);
> >
> > - return vcpu;
> > + return 0;
> > +
> > out_free_sie_block:
> > free_page((unsigned long)(vcpu->arch.sie_block));
> > -out_uninit_vcpu:
> > - kvm_vcpu_uninit(vcpu);
> > -out_free_cpu:
> > - kmem_cache_free(kvm_vcpu_cache, vcpu);
> > -out:
> > - return ERR_PTR(rc);
> > + return rc;
>
> This is getting a bit hard to follow across the patches, but I think rc
> is now only set for ucontrol guests. So this looks correct right now,
> but feels a bit brittle... should we maybe init rc to 0 and always
> return rc instead?

Yes, but only for a few patches until kvm_s390_vcpu_setup() is introduced,
at which point @rc is unconditionally set at the end.

rc = kvm_s390_vcpu_setup(vcpu);
if (rc)
goto out_free_ucontrol_gmap;
return 0;

My personal preference is to use "return 0;" when the return is known to
be zero as it makes the success path obvious at a glance. I also didn't
want to intialize @rc at he beginning because then the sie_page allocation
would look a bit funky:

int rc = 0;

BUILD_BUG_ON(sizeof(struct sie_page) != 4096);
sie_page = (struct sie_page *) get_zeroed_page(GFP_KERNEL);
if (!sie_page)
return -ENOMEM;


An alternative would be to init @rc to -ENOMEM:

int rc = -ENOMEM;

BUILD_BUG_ON(sizeof(struct sie_page) != 4096);
sie_page = (struct sie_page *) get_zeroed_page(GFP_KERNEL);
if (!sie_page)
return rc;

This would be my preference if you'd prefer to init @rc right away,
especially if __kvm_ucontrol_vcpu_init() is open coded here (discussion
in patch 35, "KVM: s390: Manually invoke vcpu setup during
kvm_arch_vcpu_create()", e.g.:

int rc = -ENOMEM;

BUILD_BUG_ON(sizeof(struct sie_page) != 4096);
sie_page = (struct sie_page *) get_zeroed_page(GFP_KERNEL);
if (!sie_page)
return rc;

...

if (kvm_is_ucontrol(vcpu->kvm)) {
vcpu->arch.gmap = gmap_create(current->mm, -1UL);
if (!vcpu->arch.gmap)
goto out_free_sie_block;
vcpu->arch.gmap->private = vcpu->kvm;
}

VM_EVENT(kvm, 3, "create cpu %d at 0x%pK, sie block at 0x%pK", id, vcpu,
vcpu->arch.sie_block);
trace_kvm_s390_create_vcpu(id, vcpu, vcpu->arch.sie_block);

rc = kvm_s390_vcpu_setup(vcpu);
if (rc)
goto out_free_ucontrol_gmap;
return 0;

out_free_ucontrol_gmap:
if (kvm_is_ucontrol(vcpu->kvm))
gmap_remove(vcpu->arch.gmap);
out_free_sie_block:
free_page((unsigned long)(vcpu->arch.sie_block));
return rc;

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