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

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

>>> On 19.06.15 at 20:44, <boris.ostrovsky@oracle.com> wrote:
> Add runtime interface for setting PMU mode and flags. Three main modes are
> provided:
> * XENPMU_MODE_OFF:  PMU is not virtualized
> * XENPMU_MODE_SELF: Guests can access PMU MSRs and receive PMU interrupts.
> * XENPMU_MODE_HV: Same as XENPMU_MODE_SELF for non-proviledged guests, dom0
>   can profile itself and the hypervisor.
> 
> Note that PMU modes are different from what can be provided at Xen's boot 
> line
> with 'vpmu' argument. An 'off' (or '0') value is equivalent to 
> XENPMU_MODE_OFF.
> Any other value, on the other hand, will cause VPMU mode to be set to
> XENPMU_MODE_SELF during boot.
> 
> For feature flags only Intel's BTS is currently supported.
> 
> Mode and flags are set via HYPERVISOR_xenpmu_op hypercall.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>

Acked-by: Jan Beulich <jbeulich@suse.com>
with one minor comment

> +    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).

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.

Jan