Re: [Xen-devel] [PATCH v14 for-xen-4.5 17/21] x86/VPMU: Handle PMU interrupts for PV guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.10.14 at 18:08, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 10/28/2014 05:30 AM, Jan Beulich wrote:
>>>>> On 27.10.14 at 20:43, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 10/27/2014 12:54 PM, Jan Beulich wrote:
>>>>>>> On 17.10.14 at 23:18, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> --- a/xen/arch/x86/hvm/vpmu.c
>>>>> +++ b/xen/arch/x86/hvm/vpmu.c
>>>>> @@ -81,46 +81,206 @@ static void __init parse_vpmu_param(char *s)
>>>>>    
>>>>>    void vpmu_lvtpc_update(uint32_t val)
>>>>>    {
>>>>> -    struct vpmu_struct *vpmu = vcpu_vpmu(current);
>>>>> +    struct vcpu *curr = current;
>>>>> +    struct vpmu_struct *vpmu = vcpu_vpmu(curr);
>>>>>    
>>>>>        vpmu->hw_lapic_lvtpc = PMU_APIC_VECTOR | (val & APIC_LVT_MASKED);
>>>>> -    apic_write(APIC_LVTPC, vpmu->hw_lapic_lvtpc);
>>>>> +
>>>>> +    /* Postpone APIC updates for PV(H) guests if PMU interrupt is pending
>>> */
>>>>> +    if ( is_hvm_vcpu(curr) || !vpmu->xenpmu_data ||
>>>>> +         !(vpmu->xenpmu_data->pmu.pmu_flags & PMU_CACHED) )
>>>> Isn't this the pointer that pvpmu_finish() deallocates (and needs to
>>>> clear? If so, there's a race between it being cleared and used. If you
>>>> need it in places like this, perhaps you'd be better off never clearing
>>>> it and leaving the MFN allocated?
>>> This will be one of the places that check for VPMU_CONTEXT_ALLOCATED.
>> But how will adding this check make this race free?
> 
> This VCPU is paused while we are tearing down its pvpmu so we can't be 
> in the middle if this (or other, such as vpmu_do_msr()) hypercall, can we?
> 
> Or it's not paused if the VCPU is the same one that is doing the 
> teardown. In which case we certainly are not in this hypercall.
> 
> So, in fact, the check for VPMU_CONTEXT_ALLOCATED may not be needed at all.

Ah, sorry - I managed to overlook the pausing in pvpmu_finish().

Jan


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