Re: [RFC PATCH] arm64: swiotlb: dma: its: Ensure shared buffers are properly aligned

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Thomas Gleixner <tglx@linutronix.de> writes:

> On Fri, Sep 05 2025 at 11:24, Aneesh Kumar K. V. wrote:
>> When running with private memory guests, the guest kernel must allocate
>> memory with specific constraints when sharing it with the hypervisor.
>>
>> These shared memory buffers are also accessed by the host kernel, which
>> means they must be aligned to the host kernel's page size.
>>
>> This patch introduces a new helper, arch_shared_mem_alignment(), which
>
> # git grep "This patch" Documentation/process/
>
>> can be used to enforce proper alignment of shared buffers.
>>
>> The actual implementation of arch_shared_mem_alignment() is deferred
>> to a follow-up patch.
>>
>> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>> ---
>>  arch/arm64/include/asm/mem_encrypt.h |  6 ++++++
>>  arch/arm64/mm/init.c                 |  4 +++-
>>  arch/arm64/mm/mem_encrypt.c          |  6 ++++++
>>  drivers/irqchip/irq-gic-v3-its.c     |  8 ++++++--
>>  include/linux/mem_encrypt.h          |  7 +++++++
>>  include/linux/swiotlb.h              |  7 ++++---
>>  kernel/dma/direct.c                  |  7 +++++++
>>  kernel/dma/pool.c                    |  1 +
>>  kernel/dma/swiotlb.c                 | 28 +++++++++++++++++-----------
>
> This does too many things at once and breaks all swiotlb users except
> arm64. Seriously?
>
>> -void swiotlb_init(bool addressing_limited, unsigned int flags);
>> +void swiotlb_init(bool addressing_limited, unsigned int alignment, unsigned int flags);
>
> Why do you need this alignment argument in the first place?
>
> In quite some other places you use arch_shared_mem_alignment(), which
> defaults to PAGE_SIZE if the architecture does not implement it's own
> variant. What's preventing you from using that in the init functions as
> well?
>

This patch is required to ensure that a guest using a 4K page size can
safely share pages with a non-secure host that uses a 64K page size.
Without this, the non-secure host may inadvertently map protected
memory.

Memory attribute tracking in the non-secure host (via
set_guest_memory_attributes()) operates in units of the host's
PAGE_SIZE. Attempting to set memory attributes at 4K granularity—when
the host uses 64K pages—will fail with -EINVAL. This makes it impossible
to correctly manage mixed-private/shared regions without enforcing
alignment between guest and host page sizes or introducing finer-grained
handling.


-aneesh