RE: [PATCH v6 3/6] arm64: hyperv: Add per-CPU RSI host call infrastructure for CCA Realms

[Michael Kelley <mhklinux@outlook.com>]

From: Kameron Carr <kameroncarr@linux.microsoft.com> Sent: Friday, October 30, 2026 5:14 PM
> 
> Arm CCA Realms cannot issue Hyper-V hypercalls via HVC; the guest must
> route them through the RSI_HOST_CALL interface, which takes the IPA of a
> per-CPU rsi_host_call structure as its argument.
> 
> Add hv_hostcall_array as a per-CPU struct array and allocate it during
> hyperv_init().

A quibble: This statement is a bit misleading. I think you mean it is a
global array with an entry for each CPU.  The term "per-CPU" typically
means something allocated out of the per-CPU area.

Michael
 
> The allocation is gated on is_realm_world() so non-Realm
> arm64 Hyper-V guests pay no memory cost.
> 
> Reviewed-by: Michael Kelley <mhklinux@outlook.com>
> Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
> ---
>  arch/arm64/hyperv/mshyperv.c      | 27 +++++++++++++++++++++++++--
>  arch/arm64/include/asm/mshyperv.h |  4 ++++
>  2 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> index 82572c5..46ab2da 100644
> --- a/arch/arm64/hyperv/mshyperv.c
> +++ b/arch/arm64/hyperv/mshyperv.c
> @@ -14,12 +14,16 @@
>  #include <linux/arm-rsi-cmds.h>
>  #include <linux/export.h>
>  #include <linux/errno.h>
> +#include <linux/slab.h>
>  #include <linux/version.h>
>  #include <linux/cpuhotplug.h>
>  #include <asm/mshyperv.h>
> 
>  static bool hyperv_initialized;
> 
> +struct rsi_host_call *hv_hostcall_array;
> +EXPORT_SYMBOL_GPL(hv_hostcall_array);
> +
>  int hv_get_hypervisor_version(union hv_hypervisor_version_info *info)
>  {
>  	hv_get_vpreg_128(HV_REGISTER_HYPERVISOR_VERSION,
> @@ -110,6 +114,20 @@ static int __init hyperv_init(void)
>  	if (!hyperv_detect_via_acpi() && !hyperv_detect_via_smccc())
>  		return 0;
> 
> +	/*
> +	 * The RSI host-call buffers are only ever used when
> +	 * is_realm_world() is true. Skip the allocation on non-Realm
> +	 * guests. A single contiguous array of nr_cpu_ids entries is
> +	 * allocated; each CPU indexes into it by its processor ID.
> +	 */
> +	if (is_realm_world()) {
> +		hv_hostcall_array = kcalloc(nr_cpu_ids,
> +					    sizeof(struct rsi_host_call),
> +					    GFP_KERNEL);
> +		if (!hv_hostcall_array)
> +			return -ENOMEM;
> +	}
> +
>  	/* Setup the guest ID */
>  	guest_id = hv_generate_guest_id(LINUX_VERSION_CODE);
>  	hv_set_vpreg(HV_REGISTER_GUEST_OS_ID, guest_id);
> @@ -131,13 +149,13 @@ static int __init hyperv_init(void)
> 
>  	ret = hv_common_init();
>  	if (ret)
> -		return ret;
> +		goto free_hostcall_mem;
> 
>  	ret = cpuhp_setup_state(CPUHP_AP_HYPERV_ONLINE,
> "arm64/hyperv_init:online",
>  				hv_common_cpu_init, hv_common_cpu_die);
>  	if (ret < 0) {
>  		hv_common_free();
> -		return ret;
> +		goto free_hostcall_mem;
>  	}
> 
>  	if (ms_hyperv.priv_high & HV_ACCESS_PARTITION_ID)
> @@ -150,6 +168,11 @@ static int __init hyperv_init(void)
> 
>  	hyperv_initialized = true;
>  	return 0;
> +
> +free_hostcall_mem:
> +	kfree(hv_hostcall_array);
> +	hv_hostcall_array = NULL;
> +	return ret;
>  }
> 
>  early_initcall(hyperv_init);
> diff --git a/arch/arm64/include/asm/mshyperv.h
> b/arch/arm64/include/asm/mshyperv.h
> index b721d31..aed0aa5 100644
> --- a/arch/arm64/include/asm/mshyperv.h
> +++ b/arch/arm64/include/asm/mshyperv.h
> @@ -61,6 +61,10 @@ static inline u64 hv_get_non_nested_msr(unsigned int reg)
>  				ARM_SMCCC_OWNER_VENDOR_HYP,	\
>  				HV_SMCCC_FUNC_NUMBER)
> 
> +/* Per-CPU-indexed RSI host call structures for CCA Realms */
> +struct rsi_host_call;
> +extern struct rsi_host_call *hv_hostcall_array;
> +
>  #include <asm-generic/mshyperv.h>
> 
>  #endif
> --
> 2.45.4
>