linux-kernel - Re: [PATCH v5 07/10] Drivers: hv: Introduce per-cpu event ring tail

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Message-ID: <6ba21efb-7065-44d7-9cee-265a0c137f0c@linux.microsoft.com>
Date: Thu, 13 Mar 2025 08:56:18 -0700
From: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To: Tianyu Lan <ltykernel@...il.com>
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Subject: Re: [PATCH v5 07/10] Drivers: hv: Introduce per-cpu event ring tail

On 3/13/2025 12:34 AM, Tianyu Lan wrote:
> On Thu, Mar 13, 2025 at 3:45 AM Nuno Das Neves
> <nunodasneves@...ux.microsoft.com> wrote:
>>
>> On 3/10/2025 6:01 AM, Tianyu Lan wrote:
>>> On Thu, Feb 27, 2025 at 7:09 AM Nuno Das Neves
>>> <nunodasneves@...ux.microsoft.com> wrote:
>>>>
>>>> Add a pointer hv_synic_eventring_tail to track the tail pointer for the
>>>> SynIC event ring buffer for each SINT.
>>>>
>>>> This will be used by the mshv driver, but must be tracked independently
>>>> since the driver module could be removed and re-inserted.
>>>>
>>>> Signed-off-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
>>>> Reviewed-by: Wei Liu <wei.liu@...nel.org>
>>>
>>> It's better to expose a function to check the tail instead of exposing
>>> hv_synic_eventring_tail directly.
>>>
>> What is the advantage of using a function for this? We need to both set
>> and get the tail.
> 
> We may add lock or check to avoid race conditions and this depends on the
> user case. This is why I want to see how mshv driver uses it.
> 
>>
>>> BTW, how does mshv driver use hv_synic_eventring_tail? Which patch
>>> uses it in this series?
>>>
>> This variable stores indices into the synic eventring page (one for each
>> SINT, and per-cpu). Each SINT has a ringbuffer of u32 messages. The tail
>> index points to the latest one.
>>
>> This is only used for doorbell messages today. The message in this case is
>> a port number which is used to lookup and invoke a callback, which signals
>> ioeventfd(s), to notify the VMM of a guest MMIO write.
>>
>> It is used in patch 10.
> 
> I found "extern u8 __percpu **hv_synic_eventring_tail;" in the
> drivers/hv/mshv_root.h of patch 10.
> I seem to miss the code to use it.
> 
> +int hv_call_unmap_stat_page(enum hv_stats_object_type type,
> +                           const union hv_stats_object_identity *identity);
> +int hv_call_modify_spa_host_access(u64 partition_id, struct page **pages,
> +                                  u64 page_struct_count, u32 host_access,
> +                                  u32 flags, u8 acquire);
> +
> +extern struct mshv_root mshv_root;
> +extern enum hv_scheduler_type hv_scheduler_type;
> +extern u8 __percpu **hv_synic_eventring_tail;
> +
> +#endif /* _MSHV_ROOT_H_ */
> 

It is used in mshv_synic.c in synic_event_ring_get_queued_port():

diff --git a/drivers/hv/mshv_synic.c b/drivers/hv/mshv_synic.c
new file mode 100644
index 000000000000..e7782f92e339
--- /dev/null
+++ b/drivers/hv/mshv_synic.c
@@ -0,0 +1,665 @@
<snip>
+static u32 synic_event_ring_get_queued_port(u32 sint_index)
+{
+	struct hv_synic_event_ring_page **event_ring_page;
+	volatile struct hv_synic_event_ring *ring;
+	struct hv_synic_pages *spages;
+	u8 **synic_eventring_tail;
+	u32 message;
+	u8 tail;
+
+	spages = this_cpu_ptr(mshv_root.synic_pages);
+	event_ring_page = &spages->synic_event_ring_page;
+	synic_eventring_tail = (u8 **)this_cpu_ptr(hv_synic_eventring_tail);
+	tail = (*synic_eventring_tail)[sint_index];