Re: [PATCH v2 10/14] arm64: Force device mappings to be non-secure shared

Suzuki K Poulose
2024-05-15T11:00:54Z

On 15/05/2024 10:01, Catalin Marinas wrote:
> On Fri, Apr 12, 2024 at 09:42:09AM +0100, Steven Price wrote:
>> From: Suzuki K Poulose <suzuki.poulose@arm.com>
>>
>> Device mappings (currently) need to be emulated by the VMM so must be
>> mapped shared with the host.
>
> You say "currently". What's the plan when the device is not emulated?
> How would the guest distinguish what's emulated and what's not to avoid
> setting the PROT_NS_SHARED bit?

Arm CCA plans to add support for passing through real devices,
which support PCI-TDISP protocol. This would involve the Realm
authenticating the device and explicitly requesting "protected"
mapping *after* the verification (with the help of RMM).

>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>> arch/arm64/include/asm/pgtable.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index f5376bd567a1..db71c564ec21 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -598,7 +598,7 @@ static inline void set_pud_at(struct mm_struct *mm, unsigned long addr,
>> #define pgprot_writecombine(prot) \
>> __pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_NORMAL_NC) | PTE_PXN | PTE_UXN)
>> #define pgprot_device(prot) \
>> - __pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_DEVICE_nGnRE) | PTE_PXN | PTE_UXN)
>> + __pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_DEVICE_nGnRE) | PTE_PXN | PTE_UXN | PROT_NS_SHARED)
>
> This pgprot_device() is not the only one used to map device resources.
> pgprot_writecombine() is another commonly macro. It feels like a hack to
> plug one but not the other and without any way for the guest to figure
> out what's emulated.

Agree. I have been exploring hooking this into ioremap_prot() where we
could apply the attribute accordingly. We will change it in the next
version.

>
> Can the DT actually place those emulated ranges in the higher IPA space
> so that we avoid randomly adding this attribute for devices?

It can, but then we kind of break the "Realm" view of the IPA space.
i.e., right now it only knows about the "lower IPA" half and uses the
top bit as a protection attr to access the IPA as shared.

Expanding IPA size view kind of breaks "sharing memory", where, we
must "use a different PA" for a page that is now shared.

Suzuki