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



On 06/23/2015 04:26 AM, Jan Beulich wrote:
On 22.06.15 at 18:10, <boris.ostrovsky@xxxxxxxxxx> 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.

Oh, right, I was only looking at the change that you made to 'if', not to the 'esle'. Yes, that's good.

-boris


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