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Re: [PATCH v2 3/7] xen/evtchn: restrict the maximum number of evtchn supported for domUs
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Rahul Singh <Rahul.Singh@xxxxxxx>
- Date: Tue, 23 Aug 2022 10:39:07 +0000
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- Cc: Julien Grall <julien@xxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 23 Aug 2022 10:39:25 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH v2 3/7] xen/evtchn: restrict the maximum number of evtchn supported for domUs
Hi Jan,
> On 23 Aug 2022, at 9:29 am, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 23.08.2022 09:56, Julien Grall wrote:
>> On 22/08/2022 14:49, Jan Beulich wrote:
>>> On 19.08.2022 12:02, Rahul Singh wrote:
>>>> --- a/xen/arch/arm/domain_build.c
>>>> +++ b/xen/arch/arm/domain_build.c
>>>> @@ -3277,7 +3277,7 @@ 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),
>>>> --- a/xen/include/xen/sched.h
>>>> +++ b/xen/include/xen/sched.h
>>>> @@ -76,6 +76,9 @@ 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)
>>>>
>>>> +/* Maximum number of event channels supported for domUs. */
>>>> +#define MAX_EVTCHNS_PORT 4096
>>>
>>> I'm afraid the variable name doesn't express its purpose, and the
>>> comment also claims wider applicability than is actually the case.
>>> It's also not clear whether the constant really needs to live in
>>> the already heavily overloaded xen/sched.h.
>>
>> IMHO, I think the value would be better hardcoded with an explanation on
>> top how we chose the default value.
>
> Indeed that might be best, at least as long as no 2nd party appears.
> What I was actually considering a valid reason for having a constant
> in a header was the case of other arches also wanting to support
> dom0less, at which point they likely ought to use the same value
> without needing to duplicate any commentary or alike.
If everyone is okay I will modify the patch as below:
diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index 3fd1186b53..fde133cd94 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -3277,7 +3277,13 @@ 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,
+ /*
+ * The default of 1023 should be sufficient for domUs guests
+ * because on ARM we don't bind physical interrupts to event
+ * channels. The only use of the evtchn port is inter-domain
+ * communications.
+ */
+ .max_evtchn_port = 1023,
.max_grant_frames = -1,
.max_maptrack_frames = -1,
.grant_opts = XEN_DOMCTL_GRANT_version(opt_gnttab_max_version),
Regards,
Rahul
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