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

[Nuno Das Neves <nunodasneves@linux.microsoft.com>]

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.

> 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.

Thanks
Nuno
 
> Thanks.
> 
> 
>> ---
>>  drivers/hv/hv_common.c | 34 ++++++++++++++++++++++++++++++++--
>>  1 file changed, 32 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
>> index 252fd66ad4db..2763cb6d3678 100644
>> --- a/drivers/hv/hv_common.c
>> +++ b/drivers/hv/hv_common.c
>> @@ -68,6 +68,16 @@ static void hv_kmsg_dump_unregister(void);
>>
>>  static struct ctl_table_header *hv_ctl_table_hdr;
>>
>> +/*
>> + * Per-cpu array holding the tail pointer for the SynIC event ring buffer
>> + * for each SINT.
>> + *
>> + * We cannot maintain this in mshv driver because the tail pointer should
>> + * persist even if the mshv driver is unloaded.
>> + */
>> +u8 __percpu **hv_synic_eventring_tail;
>> +EXPORT_SYMBOL_GPL(hv_synic_eventring_tail);
>> +
>>  /*
>>   * Hyper-V specific initialization and shutdown code that is
>>   * common across all architectures.  Called from architecture
>> @@ -90,6 +100,9 @@ void __init hv_common_free(void)
>>
>>         free_percpu(hyperv_pcpu_input_arg);
>>         hyperv_pcpu_input_arg = NULL;
>> +
>> +       free_percpu(hv_synic_eventring_tail);
>> +       hv_synic_eventring_tail = NULL;
>>  }
>>
>>  /*
>> @@ -372,6 +385,11 @@ int __init hv_common_init(void)
>>                 BUG_ON(!hyperv_pcpu_output_arg);
>>         }
>>
>> +       if (hv_root_partition()) {
>> +               hv_synic_eventring_tail = alloc_percpu(u8 *);
>> +               BUG_ON(hv_synic_eventring_tail == NULL);
>> +       }
>> +
>>         hv_vp_index = kmalloc_array(nr_cpu_ids, sizeof(*hv_vp_index),
>>                                     GFP_KERNEL);
>>         if (!hv_vp_index) {
>> @@ -460,6 +478,7 @@ void __init ms_hyperv_late_init(void)
>>  int hv_common_cpu_init(unsigned int cpu)
>>  {
>>         void **inputarg, **outputarg;
>> +       u8 **synic_eventring_tail;
>>         u64 msr_vp_index;
>>         gfp_t flags;
>>         const int pgcount = hv_output_page_exists() ? 2 : 1;
>> @@ -472,8 +491,8 @@ int hv_common_cpu_init(unsigned int cpu)
>>         inputarg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
>>
>>         /*
>> -        * hyperv_pcpu_input_arg and hyperv_pcpu_output_arg memory is already
>> -        * allocated if this CPU was previously online and then taken offline
>> +        * The per-cpu memory is already allocated if this CPU was previously
>> +        * online and then taken offline
>>          */
>>         if (!*inputarg) {
>>                 mem = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);
>> @@ -485,6 +504,17 @@ int hv_common_cpu_init(unsigned int cpu)
>>                         *outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
>>                 }
>>
>> +               if (hv_root_partition()) {
>> +                       synic_eventring_tail = (u8 **)this_cpu_ptr(hv_synic_eventring_tail);
>> +                       *synic_eventring_tail = kcalloc(HV_SYNIC_SINT_COUNT,
>> +                                                       sizeof(u8), flags);
>> +
>> +                       if (unlikely(!*synic_eventring_tail)) {
>> +                               kfree(mem);
>> +                               return -ENOMEM;
>> +                       }
>> +               }
>> +
>>                 if (!ms_hyperv.paravisor_present &&
>>                     (hv_isolation_type_snp() || hv_isolation_type_tdx())) {
>>                         ret = set_memory_decrypted((unsigned long)mem, pgcount);
>> --
>> 2.34.1
>>
>>
> 
>