Re: [PATCH v4 05/15] arm64: Mark all I/O as non-secure shared

Suzuki K Poulose
2024-07-09T12:54:20Z

Hi Will

On 09/07/2024 12:39, Will Deacon wrote:
> On Mon, Jul 01, 2024 at 10:54:55AM +0100, Steven Price wrote:
>> All I/O is by default considered non-secure for realms. As such
>> mark them as shared with the host.
>>
>> Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>> Changes since v3:
>> * Add PROT_NS_SHARED to FIXMAP_PAGE_IO rather than overriding
>> set_fixmap_io() with a custom function.
>> * Modify ioreamp_cache() to specify PROT_NS_SHARED too.
>> ---
>> arch/arm64/include/asm/fixmap.h | 2 +-
>> arch/arm64/include/asm/io.h | 8 ++++----
>> 2 files changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h
>> index 87e307804b99..f2c5e653562e 100644
>> --- a/arch/arm64/include/asm/fixmap.h
>> +++ b/arch/arm64/include/asm/fixmap.h
>> @@ -98,7 +98,7 @@ enum fixed_addresses {
>> #define FIXADDR_TOT_SIZE (__end_of_fixed_addresses << PAGE_SHIFT)
>> #define FIXADDR_TOT_START (FIXADDR_TOP - FIXADDR_TOT_SIZE)
>>
>> -#define FIXMAP_PAGE_IO __pgprot(PROT_DEVICE_nGnRE)
>> +#define FIXMAP_PAGE_IO __pgprot(PROT_DEVICE_nGnRE | PROT_NS_SHARED)
>>
>> void __init early_fixmap_init(void);
>>
>> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
>> index 4ff0ae3f6d66..07fc1801c6ad 100644
>> --- a/arch/arm64/include/asm/io.h
>> +++ b/arch/arm64/include/asm/io.h
>> @@ -277,12 +277,12 @@ static inline void __const_iowrite64_copy(void __iomem *to, const void *from,
>>
>> #define ioremap_prot ioremap_prot
>>
>> -#define _PAGE_IOREMAP PROT_DEVICE_nGnRE
>> +#define _PAGE_IOREMAP (PROT_DEVICE_nGnRE | PROT_NS_SHARED)
>>
>> #define ioremap_wc(addr, size) \
>> - ioremap_prot((addr), (size), PROT_NORMAL_NC)
>> + ioremap_prot((addr), (size), (PROT_NORMAL_NC | PROT_NS_SHARED))
>> #define ioremap_np(addr, size) \
>> - ioremap_prot((addr), (size), PROT_DEVICE_nGnRnE)
>> + ioremap_prot((addr), (size), (PROT_DEVICE_nGnRnE | PROT_NS_SHARED))
>
> Hmm. I do wonder whether you've pushed the PROT_NS_SHARED too far here.
>
> There's nothing _architecturally_ special about the top address bit.
> Even if the RSI divides the IPA space in half, the CPU doesn't give two
> hoots about it in the hardware. In which case, it feels wrong to bake
> PROT_NS_SHARED into ioremap_prot -- it feels much better to me if the
> ioremap() code OR'd that into the physical address when passing it down

Actually we would like to push the decision of applying the
"pgprot_decrypted" vs pgprot_encrypted into ioremap_prot(), rather
than sprinkling every user of ioremap_prot().

This could be made depending on the address that is passed on to the
ioremap_prot(). I guess we would need explicit requests from the callers
to add "encrypted vs decrypted". Is that what you guys are looking at ?

>
> There's a selfish side of that argument, in that we need to hook
> ioremap() for pKVM protected guests, but I do genuinely feel that
> treating address bits as protection bits is arbitrary and doesn't belong
> in these low-level definitions. In a similar vein, AMD has its
> sme_{set,clr}() macros that operate on the PA (e.g. via dma_to_phys()),
> which feels like a more accurate abstraction to me.

I believe that doesn't solve all the problems. They do have a hook in
__ioremap_caller() that implicitly applies pgprot_{en,de}crypted
depending on other info.

Cheers
Suzuki

>
> Will