Re: [PATCH v25 04/15] x86/VPMU: Interface for setting PMU mode and flags

["Jan Beulich" <JBeulich@suse.com>]

>>> On 22.06.15 at 18:10, <boris.ostrovsky@oracle.com> wrote:
> On 06/22/2015 11:10 AM, Jan Beulich wrote:
>>
>>> +    switch ( op )
>>> +    {
>>> +    case XENPMU_mode_set:
>>> +    {
>>> +        if ( (pmu_params.val & ~(XENPMU_MODE_SELF | XENPMU_MODE_HV)) ||
>>> +             (hweight64(pmu_params.val) > 1) )
>>> +            return -EINVAL;
>>> +
>>> +        /* 32-bit dom0 can only sample itself. */
>>> +        if ( is_pv_32bit_vcpu(current) && (pmu_params.val & XENPMU_MODE_HV) )
>>> +            return -EINVAL;
>>> +
>>> +        spin_lock(&vpmu_lock);
>>> +
>>> +        /*
>>> +         * We can always safely switch between XENPMU_MODE_SELF and
>>> +         * XENPMU_MODE_HV while other VPMUs are active.
>>> +         */
>>> +        if ( (vpmu_count == 0) || (vpmu_mode == pmu_params.val) ||
>>> +             ((vpmu_mode ^ pmu_params.val) ==
>>> +              (XENPMU_MODE_SELF | XENPMU_MODE_HV)) )
>>> +            vpmu_mode = pmu_params.val;
>>> +        else
>>> +        {
>> I think this would better be
>>
>>          if ( (vpmu_count == 0) ||
>>               ((vpmu_mode ^ pmu_params.val) ==
>>                (XENPMU_MODE_SELF | XENPMU_MODE_HV)) )
>>              vpmu_mode = pmu_params.val;
>>          else if ( vpmu_mode != pmu_params.val )
>>          {
>>
>> But there's no need to re-submit just because of this (it could be
>> changed upon commit if you agree).
> 
> This will generate an error (with a message to the log) even if we keep 
> the mode unchanged, something that I wanted to avoid. (Same thing for 
> XENPMU_feature_set, which is what Kevin mentioned last time).

I don't see this: The only difference to the code you have is -
afaics - that my variant avoids the pointless write to vpmu_mode.

>> Or wait - I just checked again, and while I thought this was long
>> addressed I still don't seen struct xen_pmu_params "pad" field
>> being checked to be zero as input, and being stored as zero when
>> only an output. Did this get lost? Did I forget about a reason why
>> this isn't being done? Unless the latter is the case, the ack above
>> is dependent upon this getting fixed.
> 
> Yes, we did talk about this and as result I added major version check 
> (which I should have had there anyway): if we decide to start using this 
> field we'd need to increment the major version.

Okay then.

Jan