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

Nuno Das Neves
2025-03-12T19:45:04Z

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
>>
>>
>
>