Re: [PATCH 2/8] xen/evtchn: modify evtchn_alloc_unbound to allocate specified port

[Rahul Singh <Rahul.Singh@xxxxxxx>]

Hi ,

> On 13 Jul 2022, at 1:29 pm, Julien Grall <julien@xxxxxxx> wrote:
> 
> 
> 
> On 13/07/2022 13:12, Bertrand Marquis wrote:
>>> On 13 Jul 2022, at 12:31, Julien Grall <julien@xxxxxxx> wrote:
>>>> I can't
>>>> see why it would be wrong to have a more tight limit on static ports
>>>> than on traditional ("dynamic") ones. Even if only to make sure so
>>>> many dynamic ones are left.
>>> 
>>> This is similar to Xen forbidding to close a static port: it is not the 
>>> hypervisor business to check that there are enough event channel ports 
>>> freed for dynamic allocation.
>> On other side we need to be cautious not to add too much complexity in the 
>> code by trying to make things always magically work.
>> If you want Xen to be accessible to non expert by magically working all the 
>> time, there would be a lot of work to do.
> 
> It is not clear to me whether you are referring to a developper or admin here.
> 
> On the admin side, we need to make sure they have an easy way to configure 
> event channels. One knob is always going to easier than two knobs.
> 
> On the developper side, this could be resolved by better documentation in the 
> code/interface.
> 
> Cheers,

To conclude the discussion, If everyone agree I will add the below patch or 
similar in the next version to restrict the
max number of evtchn supported as suggested.


diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index 532e50e321..a8c5825a4f 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -3422,7 +3422,7 @@ void __init create_domUs(void)
         struct xen_domctl_createdomain d_cfg = {
             .arch.gic_version = XEN_DOMCTL_CONFIG_GIC_NATIVE,
             .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap,
-            .max_evtchn_port = -1,
+            .max_evtchn_port = MAX_EVTCHNS_PORT,
             .max_grant_frames = -1,
             .max_maptrack_frames = -1,
             .grant_opts = XEN_DOMCTL_GRANT_version(opt_gnttab_max_version),
@@ -3582,7 +3582,7 @@ void __init create_dom0(void)
     struct domain *dom0;
     struct xen_domctl_createdomain dom0_cfg = {
         .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap,
-        .max_evtchn_port = -1,
+        .max_evtchn_port = MAX_EVTCHNS_PORT,
         .max_grant_frames = gnttab_dom0_frames(),
         .max_maptrack_frames = -1,
         .grant_opts = XEN_DOMCTL_GRANT_version(opt_gnttab_max_version),
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index f08b07b8de..b1f95fbe1a 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -763,7 +763,7 @@ static struct domain *__init create_dom0(const module_t 
*image,
 {
     struct xen_domctl_createdomain dom0_cfg = {
         .flags = IS_ENABLED(CONFIG_TBOOT) ? XEN_DOMCTL_CDF_s3_integrity : 0,
-        .max_evtchn_port = -1,
+        .max_evtchn_port = MAX_EVTCHNS_PORT,
         .max_grant_frames = -1,
         .max_maptrack_frames = -1,
         .grant_opts = XEN_DOMCTL_GRANT_version(opt_gnttab_max_version),
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index d6c029020f..783359f733 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -76,6 +76,8 @@ extern domid_t hardware_domid;
 /* Maximum number of event channels for any ABI. */
 #define MAX_NR_EVTCHNS MAX(EVTCHN_2L_NR_CHANNELS, EVTCHN_FIFO_NR_CHANNELS)
 
+#define MAX_EVTCHNS_PORT 4096
+
 #define EVTCHNS_PER_BUCKET (PAGE_SIZE / next_power_of_2(sizeof(struct evtchn)))
 #define EVTCHNS_PER_GROUP  (BUCKETS_PER_GROUP * EVTCHNS_PER_BUCKET)
 #define NR_EVTCHN_GROUPS   DIV_ROUND_UP(MAX_NR_EVTCHNS, EVTCHNS_PER_GROUP

Regards,
Rahul