Re: [PATCH v6 4/6] Drivers: hv: Mark shared memory as decrypted for CCA Realms

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

On 11/11/2026 6:43 PM, Mukesh R wrote:
> On 11/11/26 10:01, Michael Kelley wrote:
>> From: Kameron Carr <kameroncarr@linux.microsoft.com> Sent: Friday, October 30, 2026 5:14 PM
>>>
>>> In hv_common_cpu_init(), the per-CPU hypercall input/output pages need
>>> to be marked as decrypted (shared) for confidential VM isolation types.
>>> This is already done for SNP and TDX isolation; extend the same handling
>>> to Arm CCA Realm guests so that the host hypervisor can access the
>>> shared hypercall buffers.
>>>
>>> is_realm_world() is declared in include/linux/arm-rsi-cmds.h which only
>>> builds on arm64 (it includes <asm/memory.h>), so using it directly in
>>> the arch-neutral drivers/hv/hv_common.c would break the x86 build.
>>> Introduce a Hyper-V-specific helper following the established
>>> hv_isolation_type_snp() / hv_isolation_type_tdx() pattern.
>>>
>>> On architectures other than arm64 the weak default keeps the existing
>>> behaviour.
>>>
>>> Reviewed-by: Michael Kelley <mhklinux@outlook.com>
>>> Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
>>> ---
>>>   arch/arm64/hyperv/mshyperv.c   |  5 +++++
>>>   drivers/hv/hv_common.c         | 17 +++++++++++++----
>>>   include/asm-generic/mshyperv.h |  1 +
>>>   3 files changed, 19 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
>>> index 46ab2da..da6c3aa0 100644
>>> --- a/arch/arm64/hyperv/mshyperv.c
>>> +++ b/arch/arm64/hyperv/mshyperv.c
>>> @@ -182,3 +182,8 @@ bool hv_is_hyperv_initialized(void)
>>>       return hyperv_initialized;
>>>   }
>>>   EXPORT_SYMBOL_GPL(hv_is_hyperv_initialized);
>>> +
>>> +bool hv_isolation_type_cca(void)
>>> +{
>>> +    return is_realm_world();
>>> +}
>>> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
>>> index 31256cb..a60c92c 100644
>>> --- a/drivers/hv/hv_common.c
>>> +++ b/drivers/hv/hv_common.c
>>> @@ -476,6 +476,7 @@ int hv_common_cpu_init(unsigned int cpu)
>>>       u64 msr_vp_index;
>>>       gfp_t flags;
>>>       const int pgcount = hv_output_page_exists() ? 2 : 1;
>>> +    const size_t alloc_size = ALIGN((size_t)pgcount * HV_HYP_PAGE_SIZE, PAGE_SIZE);
>>>       void *mem;
>>>       int ret = 0;
>>>
>>> @@ -489,7 +490,7 @@ int hv_common_cpu_init(unsigned int cpu)
>>>        * online and then taken offline
>>>        */
>>>       if (!*inputarg) {
>>> -        mem = kmalloc_array(pgcount, HV_HYP_PAGE_SIZE, flags);
>>> +        mem = kmalloc(alloc_size, flags);
>>>           if (!mem)
>>>               return -ENOMEM;
>>>
>>> @@ -499,14 +500,16 @@ int hv_common_cpu_init(unsigned int cpu)
>>>           }
>>>
>>>           if (!ms_hyperv.paravisor_present &&
>>> -            (hv_isolation_type_snp() || hv_isolation_type_tdx())) {
>>> -            ret = set_memory_decrypted((unsigned long)mem, pgcount);
>>> +            (hv_isolation_type_snp() || hv_isolation_type_tdx() ||
>>> +             hv_isolation_type_cca())) {
>>
>> Could this big "if" statement use "hv_is_isolation_supported()" instead
>> of explicitly testing for each CoCo VM type? I ask partly because
>> Mukesh Rathor has just submitted a patch [1] that introduces a new
>> function that does something almost exactly the same as
>> hv_is_isolation_supported() and uses it here.
> 
>>> +            (hv_isolation_type_snp() || hv_isolation_type_tdx() ||
>>> +             hv_isolation_type_cca())) {
> 
> With my change, this will become:
> 
>         (hv_isolation_hw_cvm() || hv_isolation_type_cca())) {

Any reason you wouldn't change hv_isolation_hw_cvm() to

  return hv_isolation_type_snp() || hv_isolation_type_tdx() ||
    hv_isolation_type_cca()

That would seem more logical to me.

-- 
Regards,
Kameron
> 
> Thanks,
> -Mukesh
> 
> 
>> You don't provide the non-stub hv_is_isolation_supported() until
>> Patch 6 of this series, but the weak stub should allow everything to
>> build if a bisection splits this patch series.
>>
>> Michael
>>
>> [1] https://lore.kernel.org/linux-hyperv/20261111022036.351231-2-mrathor@linux.microsoft.com/
>>
>>> +            ret = set_memory_decrypted((unsigned long)mem,
>>> +                           alloc_size >> PAGE_SHIFT);
>>>               if (ret) {
>>>                   /* It may be unsafe to free 'mem' */
>>>                   return ret;
>>>               }
>>>
>>> -            memset(mem, 0x00, pgcount * HV_HYP_PAGE_SIZE);
>>> +            memset(mem, 0x00, alloc_size);
>>>           }
>>>
>>>           /*
>>> @@ -666,6 +669,12 @@ bool __weak hv_isolation_type_tdx(void)
>>>   }
>>>   EXPORT_SYMBOL_GPL(hv_isolation_type_tdx);
>>>
>>> +bool __weak hv_isolation_type_cca(void)
>>> +{
>>> +    return false;
>>> +}
>>> +EXPORT_SYMBOL_GPL(hv_isolation_type_cca);
>>> +
>>>   void __weak hv_setup_vmbus_handler(void (*handler)(void))
>>>   {
>>>   }
>>> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
>>> index bf601d6..1fa79ab 100644
>>> --- a/include/asm-generic/mshyperv.h
>>> +++ b/include/asm-generic/mshyperv.h
>>> @@ -79,6 +79,7 @@ u64 hv_do_fast_hypercall16(u16 control, u64 input1, u64
>>> input2);
>>>
>>>   bool hv_isolation_type_snp(void);
>>>   bool hv_isolation_type_tdx(void);
>>> +bool hv_isolation_type_cca(void);
>>>
>>>   /*
>>>    * On architectures where Hyper-V doesn't support AEOI (e.g., ARM64),
>>> -- 
>>> 2.45.4
>>>