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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 02/20/2015 11:27 AM, Jan Beulich wrote:
>>>> On 20.02.15 at 17:15, <boris.ostrovsky@oracle.com> wrote:
>> On 02/20/2015 09:35 AM, Jan Beulich wrote:
>>>>>> On 16.02.15 at 23:26, <boris.ostrovsky@oracle.com> 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.
> Yes, I saw that later on, but it still doesn't look well structured. Can't
> you bail early from vpmu_initialize() the first time through for PV(H)
> guests, rather than guarding the HVM invocations with is_hvm_...()?

I could but I am not sure how it would allow me to move spin_lock_init() 
to vpmu_initialize().

We are protecting xenpmu_data and it is supposed to be set before we get 
into vpmu_initialize().


>
>>>> +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).
> But it should also protect against a second pvpmu_init() on another
> pCPU.

Right, I will move 'if (v->arch.vpmu.xenpmu_data )' under the lock (and 
clean up if it is non-NULL)

-boris