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

[Jan Beulich <jbeulich@suse.com>]

On 18.11.2025 07:45, Penny, Zheng wrote:
> [Public]
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: Thursday, October 30, 2025 9:35 PM
>> To: Penny, Zheng <penny.zheng@amd.com>
>> Cc: Huang, Ray <Ray.Huang@amd.com>; oleksii.kurochko@gmail.com; Andrew
>> Cooper <andrew.cooper3@citrix.com>; Roger Pau Monné <roger.pau@citrix.com>;
>> xen-devel@lists.xenproject.org
>> 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@suse.com>
>>
>> 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:

Yes, that or indeed introducing stubs. Which one is the lesser evil I'm having
a hard time determining. Hence I'd suggest that you go with the below, unless
someone else chimes in.

Jan

> ```
> 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