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

[Mukesh R <mrathor@linux.microsoft.com>]

On 11/12/26 16:46, Mukesh R wrote:
> On 11/12/26 09:49, Kameron Carr wrote:
>> 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.
>>
> 
> We could, I just don't know if on cca, the repeat hypercall has same
> behavior. I was leaving that for the arm team. Anyways, I sent question
> to the hyp team, if they say yes, i can update my patch to include that.
> 
> Thanks,
> -Mukesh


Confirming, arm has same behavior and needs the fix also. So, we can def
expand the check to include _cca.

I just sent V1, renaming hv_isolation_hw_cvm() to hv_isolation_type_hw().
If your patch goes after mine, you just need to add to your 0004*:

diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
index 4fdc26ade1d7..67a16b4d2235 100644
--- a/arch/arm64/hyperv/mshyperv.c
+++ b/arch/arm64/hyperv/mshyperv.c
@@ -134,3 +134,8 @@ bool hv_is_hyperv_initialized(void)
         return hyperv_initialized;
  }
  EXPORT_SYMBOL_GPL(hv_is_hyperv_initialized);
+
+bool hv_isolation_type_hw(void)
+{
+       return is_realm_world();
+}


OR, you could pick mine 0001-* and make it part of your series, and I
can drop it from mine and make it dependent on yours. But mine is small
enough that it has higher chances of going in first.....

Thanks,
-Mukesh