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

[Michael Kelley <mhklinux@outlook.com>]

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.

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
>