[PATCH v2 3/6] arm64: hyperv: Add per-CPU RSI host call infrastructure for CCA Realms

[Michael Kelley]

From: Kameron Carr <kameroncarr at linux.microsoft.com> Sent: Thursday, June 25, 2026 10:35 AM
> To: kys at microsoft.com; haiyangz at microsoft.com; wei.liu at kernel.org;
> decui at microsoft.com; longli at microsoft.com
> Cc: catalin.marinas at arm.com; will at kernel.org; mark.rutland at arm.com;
> lpieralisi at kernel.org; sudeep.holla at kernel.org; arnd at arndb.de; thuth at redhat.com; linux-
> hyperv at vger.kernel.org; linux-arm-kernel at lists.infradead.org; linux-
> kernel at vger.kernel.org; linux-arch at vger.kernel.org; mhklinux at outlook.com
> Subject: [PATCH v2 3/6] arm64: hyperv: Add per-CPU RSI host call infrastructure for CCA
> Realms
> 
> 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(). The allocation is gated on is_realm_world() so non-Realm
> arm64 Hyper-V guests pay no memory cost.
> 
> Signed-off-by: Kameron Carr <kameroncarr at linux.microsoft.com>
> ---
>  arch/arm64/hyperv/mshyperv.c      | 32 ++++++++++++++++++++++++++++++-
>  arch/arm64/include/asm/mshyperv.h |  4 ++++
>  2 files changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> index 4fdc26ade1d74..7d536d7fb557e 100644
> --- a/arch/arm64/hyperv/mshyperv.c
> +++ b/arch/arm64/hyperv/mshyperv.c
> @@ -15,10 +15,15 @@
>  #include <linux/errno.h>
>  #include <linux/version.h>
>  #include <linux/cpuhotplug.h>
> +#include <linux/slab.h>
>  #include <asm/mshyperv.h>
> +#include <asm/rsi.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,
> @@ -60,6 +65,12 @@ static bool __init hyperv_detect_via_acpi(void)
> 
>  #endif
> 
> +static void hv_hostcall_free(void)
> +{
> +	kfree(hv_hostcall_array);
> +	hv_hostcall_array = NULL;
> +}
> +
>  static bool __init hyperv_detect_via_smccc(void)
>  {
>  	uuid_t hyperv_uuid = UUID_INIT(
> @@ -85,6 +96,20 @@ 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);
> @@ -106,12 +131,13 @@ 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();
> +		hv_hostcall_free();
>  		return ret;

Let me suggest a small additional simplification. For this error
path, call hv_common_free() as you have now, but then do
"goto free_hostcall_mem". At the free_hostcall_mem label, do

	kfree(hv_hostcall_array);
	hv_hostcall_array = NULL;

directly inline, and eliminate the hv_hostcall_free() helper
function. Saves about 5 lines of code overall and I think is a 
bit simpler.

>  	}
> 
> @@ -125,6 +151,10 @@ static int __init hyperv_init(void)
> 
>  	hyperv_initialized = true;
>  	return 0;
> +
> +free_hostcall_mem:
> +	hv_hostcall_free();
> +	return ret;
>  }
> 
>  early_initcall(hyperv_init);
> diff --git a/arch/arm64/include/asm/mshyperv.h b/arch/arm64/include/asm/mshyperv.h
> index b721d3134ab66..c207a3f79b99b 100644
> --- a/arch/arm64/include/asm/mshyperv.h
> +++ b/arch/arm64/include/asm/mshyperv.h
> @@ -63,4 +63,8 @@ static inline u64 hv_get_non_nested_msr(unsigned int reg)
> 
>  #include <asm-generic/mshyperv.h>
> 
> +/* Per-CPU-indexed RSI host call structures for CCA Realms */
> +struct rsi_host_call;
> +extern struct rsi_host_call *hv_hostcall_array;
> +

The intent is that the #include of asm-generic/mshyperv.h should be
last in the arch-specific version of mshyperv.h. If there's a need to go
after the #include, that's a red flag to check if some restructuring of
the definitions would be appropriate.

Unless I'm missing something, I think these new definitions can go
above the #include.

Michael

>  #endif
> --
> 2.45.4
>