Re: [Xen-devel] [PATCH v18 07/16] x86/VPMU: Initialize PMU for PV(H) guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/20/2015 09:35 AM, Jan Beulich wrote:
> > > > On 16.02.15 at 23:26, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > --- a/xen/arch/x86/domain.c
> > +++ b/xen/arch/x86/domain.c
> > @@ -437,6 +437,8 @@ int vcpu_initialise(struct vcpu *v)
> >           vmce_init_vcpu(v);
> >       }
> >   
> > +    spin_lock_init(&v->arch.vpmu.vpmu_lock);
> This would rather seem to belong into vpmu_initialize().
vpmu_initialize() is called under this lock so we can't do this.

> > @@ -503,6 +509,16 @@ int __init amd_vpmu_init(void)
> >           return -EINVAL;
> >       }
> >   
> > +    if ( sizeof(struct xen_pmu_data) +
> > +         2 * sizeof(uint64_t) * num_counters > PAGE_SIZE )
> > +    {
> > +        printk(XENLOG_WARNING
> > +               "VPMU: Register bank does not fit into VPMU shared page\n");
> > +        counters = ctrls = NULL;
> > +        num_counters = 0;
> > +        return -ENOSPC;
> > +    }
> Wouldn't it be more reasonable to simply lower num_counters in
> that case?
AMD processors in particular have fixed number of counters per family so 
a guest would expect to see all of them. (on Intel we then might get 
away by updating CPUID leaf 0xa but I am not sure it's worth doing).

> > +static int pvpmu_init(struct domain *d, xen_pmu_params_t *params)
> > +{
> > +    struct vcpu *v;
> > +    struct vpmu_struct *vpmu;
> > +    struct page_info *page;
> > +    uint64_t gfn = params->val;
> > +
> > +    if ( vpmu_mode == XENPMU_MODE_OFF )
> > +        return -EINVAL;
> > +
> > +    if ( (params->vcpu >= d->max_vcpus) || (d->vcpu == NULL) ||
> > +         (d->vcpu[params->vcpu] == NULL) )
> > +        return -EINVAL;
> > +
> > +    if ( v->arch.vpmu.xenpmu_data )
> > +        return -EINVAL;
> > +
> > +    page = get_page_from_gfn(d, gfn, NULL, P2M_ALLOC);
> > +    if ( !page )
> > +        return -EINVAL;
> > +
> > +    if ( !get_page_type(page, PGT_writable_page) )
> > +    {
> > +        put_page(page);
> > +        return -EINVAL;
> > +    }
> > +
> > +    v = d->vcpu[params->vcpu];
> > +    vpmu = vcpu_vpmu(v);
> > +    spin_lock(&vpmu->vpmu_lock);
> > +
> > +    v->arch.vpmu.xenpmu_data = __map_domain_page_global(page);
> > +    if ( !v->arch.vpmu.xenpmu_data )
> > +    {
> > +        put_page_and_type(page);
> > +        spin_unlock(&vpmu->vpmu_lock);
> > +        return -EINVAL;
> > +    }
> > +
> > +    vpmu_initialise(v);
> > +
> > +    spin_unlock(&vpmu->vpmu_lock);
> So what is this lock guarding against here? Certainly not overwriting
> of a non-NULL v->arch.vpmu.xenpmu_data (and hence leaking a
> page reference)...
This is trying to protect a race with pvmu_finish() that could clear 
xenpmu_data.
(I actually think you were the one who suggested it).

-boris

> > @@ -504,7 +590,7 @@ long do_xenpmu_op(unsigned int op, 
> > XEN_GUEST_HANDLE_PARAM(xen_pmu_params_t) arg)
> >   
> >           if ( copy_to_guest(arg, &pmu_params, 1) )
> >               return -EFAULT;
> > -        break;
> > +         break;
> ???
>
> Jan

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