RE: [PATCH v3 27/28] xen/domctl: make HVM_PARAM_IDENT_PT conditional upon CONFIG_MGMT_HYPERCALLS

["Penny, Zheng" <penny.zheng@xxxxxxx>]

[Public]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Thursday, October 30, 2025 9:35 PM
> To: Penny, Zheng <penny.zheng@xxxxxxx>
> Cc: Huang, Ray <Ray.Huang@xxxxxxx>; oleksii.kurochko@xxxxxxxxx; Andrew
> Cooper <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>;
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3 27/28] xen/domctl: make HVM_PARAM_IDENT_PT
> conditional upon CONFIG_MGMT_HYPERCALLS
>
> On 13.10.2025 12:15, Penny Zheng wrote:
> > Helper domctl_lock_{acquire,release} is domctl_lock, which
> > HVM_PARAM_IDENT_PT uses to ensure synchronization and hence being a
> toolstack-only operation.
> > So we shall make HVM_PARAM_IDENT_PT conditional upon
> > CONFIG_MGMT_HYPERCALLS, returning -EOPNOTSUPP when
> MGMT_HYPERCALLS=n.
> >
> > Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> I fear this isn't quite what I suggested. The param get/set are XSM_TARGET, 
> i.e.
> can be used by DM as well. The particular one here shouldn't be used by a DM, 
> but
> that's a different question. Similarly in principle the PVH Dom0 building 
> code should
> be able to use this path; it doesn't right now in favor of some open- coding.
>
> What iirc I did suggest was that the serialization isn't needed when no 
> domctl can
> be used to otherwise alter (relevant) guest state.

Ah, true, serialization isn't needed when MGMT_HYPERCALLS=n, as no domctl-op 
could alter the guest state at the same time.
Then maybe adding IS_ENABLED() checking is enough:
```
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 5a50721bd0..4e1b3ee5f4 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4324,7 +4324,7 @@ static int hvm_set_param(struct domain *d, uint32_t 
index, uint64_t value)
          * the domctl_lock.
          */
         rc = -ERESTART;
-        if ( !domctl_lock_acquire() )
+        if ( IS_ENABLED(CONFIG_MGMT_HYPERCALLS) && !domctl_lock_acquire() )
             break;

         rc = 0;
@@ -4334,7 +4334,8 @@ static int hvm_set_param(struct domain *d, uint32_t 
index, uint64_t value)
             paging_update_cr3(v, false);
         domain_unpause(d);

-        domctl_lock_release();
+        if ( IS_ENABLED(CONFIG_MGMT_HYPERCALLS) )
+            domctl_lock_release();
         break;
     case HVM_PARAM_DM_DOMAIN:
         /* The only value this should ever be set to is DOMID_SELF */
```

>
> Jan