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

Michael Kelley
2025-03-07T17:02:40Z

From: Nuno Das Neves <nunodasneves@linux.microsoft.com> Sent: Wednesday, February 26, 2025 3:08 PM
>
> 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>
> ---
> 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;

I think the "__percpu" is in the wrong place here. This placement
is likely to cause errors from the "sparse" tool. It should be

u8 * __percpu *hv_synic_eventring_tail;

See the way hyperv_pcpu_input_arg, for example, is defined. And
see commit db3c65bc3a13 where I fixed hyperv_pcpu_input_arg.

> +EXPORT_SYMBOL_GPL(hv_synic_eventring_tail);

The "extern" declaration for this variable is in Patch 10 of the series
in drivers/hv/mshv_root.h. I guess that's OK, but I would normally
expect to find such a declaration in the header file associated with
where the variable is defined, which in this case is mshyperv.h.
Perhaps you are trying to restrict its usage to just mshv?

> +
> /*
> * 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;
> + }
> + }
> +

Adding this code under the "if(!*inputarg)" implicitly ties the lifecycle of
synic_eventring_tail to the lifecycle of hyperv_pcpu_input_arg and
hyperv_pcpu_output_arg. Is there some logical relationship between the
two that warrants tying the lifecycles together (other than just both being
per-cpu)? hyperv_pcpu_input_arg and hyperv_pcpu_output_arg have an
unusual lifecycle management in that they aren't freed when a CPU goes
offline, as described in the comment in hv_common_cpu_die(). Does
synic_eventring_tail also need that same unusual lifecycle?

Assuming there's no logical relationship, I'm thinking synic_eventring_tail
should be managed independent of the other two. If it does need the
unusual lifecycle, make sure to add a comment in hv_common_cpu_die()
explaining why. If it doesn't need the unusual lifecycle, maybe just do
the normal thing of allocating it in hv_common_cpu_init() and freeing
it in hv_common_cpu_die().

The code as written in your patch isn't wrong and would work OK. But
the structure implies a relationship with hyperv_pcpu_*_arg that I
suspect doesn't exist.

Michael

> if (!ms_hyperv.paravisor_present &&
> (hv_isolation_type_snp() || hv_isolation_type_tdx())) {
> ret = set_memory_decrypted((unsigned long)mem, pgcount);
> --
> 2.34.1