RE: [PATCH v2 3/3] hyperv: Add CONFIG_MSHV_ROOT to gate root partition support

[Michael Kelley]

From: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx> Sent: Thursday, February 20, 2025 10:33 AM
> 
> CONFIG_MSHV_ROOT allows kernels built to run as a normal Hyper-V guest
> to exclude the root partition code, which is expected to grow
> significantly over time.
> 
> This option is a tristate so future driver code can be built as a
> (m)odule, allowing faster development iteration cycles.
> 
> If CONFIG_MSHV_ROOT is disabled, don't compile hv_proc.c, and stub
> hv_root_partition() to return false unconditionally. This allows the
> compiler to optimize away root partition code blocks since they will
> be disabled at compile time.
> 
> Signed-off-by: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx>
> ---
>  drivers/hv/Kconfig             | 16 ++++++++++++++++
>  drivers/hv/Makefile            |  3 ++-
>  include/asm-generic/mshyperv.h | 24 ++++++++++++++++++++----
>  3 files changed, 38 insertions(+), 5 deletions(-)
> 

This code LGTM, but I'll wait on my Reviewed-by: until the dependency
with Patch 2 of the series is sorted out.

Michael Kelley


> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
> index 862c47b191af..794e88e9dc6b 100644
> --- a/drivers/hv/Kconfig
> +++ b/drivers/hv/Kconfig
> @@ -55,4 +55,20 @@ config HYPERV_BALLOON
>  	help
>  	  Select this option to enable Hyper-V Balloon driver.
> 
> +config MSHV_ROOT
> +	tristate "Microsoft Hyper-V root partition support"
> +	depends on HYPERV
> +	depends on !HYPERV_VTL_MODE
> +	# The hypervisor interface operates on 4k pages. Enforcing it here
> +	# simplifies many assumptions in the root partition code.
> +	# e.g. When withdrawing memory, the hypervisor gives back 4k pages in
> +	# no particular order, making it impossible to reassemble larger pages
> +	depends on PAGE_SIZE_4KB
> +	default n
> +	help
> +	  Select this option to enable support for booting and running as root
> +	  partition on Microsoft Hyper-V.
> +
> +	  If unsure, say N.
> +
>  endmenu
> diff --git a/drivers/hv/Makefile b/drivers/hv/Makefile
> index 9afcabb3fbd2..2b8dc954b350 100644
> --- a/drivers/hv/Makefile
> +++ b/drivers/hv/Makefile
> @@ -13,4 +13,5 @@ hv_vmbus-$(CONFIG_HYPERV_TESTING)	+= hv_debugfs.o
>  hv_utils-y := hv_util.o hv_kvp.o hv_snapshot.o hv_utils_transport.o
> 
>  # Code that must be built-in
> -obj-$(subst m,y,$(CONFIG_HYPERV)) += hv_common.o hv_proc.o
> +obj-$(subst m,y,$(CONFIG_HYPERV)) += hv_common.o
> +obj-$(subst m,y,$(CONFIG_MSHV_ROOT)) += hv_proc.o
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index d2b1a8fc074c..b29357cff2f7 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -223,10 +223,6 @@ void *hv_alloc_hyperv_page(void);
>  void *hv_alloc_hyperv_zeroed_page(void);
>  void hv_free_hyperv_page(void *addr);
> 
> -int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages);
> -int hv_call_add_logical_proc(int node, u32 lp_index, u32 acpi_id);
> -int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags);
> -
>  /**
>   * hv_cpu_number_to_vp_number() - Map CPU to VP.
>   * @cpu_number: CPU number in Linux terms
> @@ -327,9 +323,29 @@ static inline enum hv_isolation_type
> hv_get_isolation_type(void)
>  }
>  #endif /* CONFIG_HYPERV */
> 
> +#if IS_ENABLED(CONFIG_MSHV_ROOT)
>  static inline bool hv_root_partition(void)
>  {
>  	return hv_current_partition_type == HV_PARTITION_TYPE_ROOT;
>  }
> +int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages);
> +int hv_call_add_logical_proc(int node, u32 lp_index, u32 acpi_id);
> +int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags);
> +
> +#else /* CONFIG_MSHV_ROOT */
> +static inline bool hv_root_partition(void) { return false; }
> +static inline int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages)
> +{
> +	return -EOPNOTSUPP;
> +}
> +static inline int hv_call_add_logical_proc(int node, u32 lp_index, u32 acpi_id)
> +{
> +	return -EOPNOTSUPP;
> +}
> +static inline int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags)
> +{
> +	return -EOPNOTSUPP;
> +}
> +#endif /* CONFIG_MSHV_ROOT */
> 
>  #endif
> --
> 2.34.1