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

[Kameron Carr <kameroncarr@linux.microsoft.com>]

On 11/11/2026 10:00 AM, Michael Kelley wrote:
> 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.

I agree. Earlier versions of the patch did use a per-CPU struct but it
was changed to a global array because the structure was small enough.

This series has already been applied to hyperv-next, but I can reach out
to Wei about editing the commit messages.

-- 
Regards,
Kameron

> 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
>>