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

Tianyu Lan
2025-03-13T16:00:56Z

On Thu, Mar 13, 2025 at 11:56 PM Nuno Das Neves
<nunodasneves@linux.microsoft.com> wrote:
>
> On 3/13/2025 12:34 AM, Tianyu Lan wrote:
> > On Thu, Mar 13, 2025 at 3:45 AM Nuno Das Neves
> > <nunodasneves@linux.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@linux.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@linux.microsoft.com>
> >>>> Reviewed-by: Wei Liu <wei.liu@kernel.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];

OK. I got it. Thanks.

--
Thanks
Tianyu Lan