Re: [PATCH v4 3/3] coco: guest: arm64: Query host IPA-change alignment via RHI

[Will Deacon <will@kernel.org>]

[+Seb for the ITS]

On Mon, Apr 27, 2026 at 12:01:08PM +0530, Aneesh Kumar K.V (Arm) wrote:
> Add the Realm Host Interface support needed to query host configuration
> from a Realm guest. Define the RHI hostconf SMCs, add rsi_host_call(), and
> use them during Realm initialization to retrieve the host IPA-change
> alignment size.
> 
> Expose that alignment through realm_get_hyp_pagesize() and
> mem_decrypt_granule_size() so shared-buffer allocation and
> encryption/decryption paths can honor the ipa change page-size requirement.
> 
> If the host reports an invalid alignment (when alginment value is not
> multiple of 4K), do not enable Realm support.
> 
> This provides the host alignment information required by the shared buffer
> alignment changes.
> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>  arch/arm64/include/asm/mem_encrypt.h |  3 ++
>  arch/arm64/include/asm/rhi.h         | 24 +++++++++++++
>  arch/arm64/include/asm/rsi.h         |  2 ++
>  arch/arm64/include/asm/rsi_cmds.h    | 10 ++++++
>  arch/arm64/include/asm/rsi_smc.h     |  7 ++++
>  arch/arm64/kernel/Makefile           |  2 +-
>  arch/arm64/kernel/rhi.c              | 54 ++++++++++++++++++++++++++++
>  arch/arm64/kernel/rsi.c              | 13 +++++++
>  arch/arm64/mm/mem_encrypt.c          |  8 +++++
>  9 files changed, 122 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/rhi.h
>  create mode 100644 arch/arm64/kernel/rhi.c

[...]

> diff --git a/arch/arm64/mm/mem_encrypt.c b/arch/arm64/mm/mem_encrypt.c
> index 38c62c9e4e74..f5d64bc29c20 100644
> --- a/arch/arm64/mm/mem_encrypt.c
> +++ b/arch/arm64/mm/mem_encrypt.c
> @@ -59,3 +59,11 @@ int set_memory_decrypted(unsigned long addr, int numpages)
>  	return crypt_ops->decrypt(addr, numpages);
>  }
>  EXPORT_SYMBOL_GPL(set_memory_decrypted);
> +
> +size_t mem_decrypt_granule_size(void)
> +{
> +	if (is_realm_world())
> +		return max(PAGE_SIZE, realm_get_hyp_pagesize());
> +	return PAGE_SIZE;

No, this should be indirected via 'struct arm64_mem_crypt_ops' because
there's nothing particularly unique to realms here. For pKVM protected
guests using a smaller page-size than the host, we'd presumably need
something similar for the ITS (where restricted-dma isn't used).

Will