Re: [PATCH v4 00/15] arm64: Support for running as a guest in Arm CCA

Steven Price
2024-08-16T16:06:29Z

On 15/08/2024 23:16, Shanker Donthineni wrote:
> Hi Steven,
>
> On 7/12/24 03:54, Matias Ezequiel Vara Larsen wrote:
>> On Mon, Jul 01, 2024 at 10:54:50AM +0100, Steven Price wrote:
>>> This series adds support for running Linux in a protected VM under the
>>> Arm Confidential Compute Architecture (CCA). This has been updated
>>> following the feedback from the v3 posting[1]. Thanks for the feedback!
>>> Individual patches have a change log. But things to highlight:
>>>
>>>   * a new patch ("firmware/psci: Add psci_early_test_conduit()") to
>>>     prevent SMC calls being made on systems which don't support them -
>>>     i.e. systems without EL2/EL3 - thanks Jean-Philippe!
>>>
>>>   * two patches dropped (overriding set_fixmap_io). Instead
>>>     FIXMAP_PAGE_IO is modified to include PROT_NS_SHARED. When support
>>>     for assigning hardware devices to a realm guest is added this will
>>>     need to be brought back in some form. But for now it's just adding
>>>     complixity and confusion for no gain.
>>>
>>>   * a new patch ("arm64: mm: Avoid TLBI when marking pages as valid")
>>>     which avoids doing an extra TLBI when doing the break-before-make.
>>>     Note that this changes the behaviour in other cases when making
>>>     memory valid. This should be safe (and saves a TLBI for those
>>> cases),
>>>     but it's a separate patch in case of regressions.
>>>
>>>   * GIC ITT allocation now uses a custom genpool-based allocator. I
>>>     expect this will be replaced with a generic way of allocating
>>>     decrypted memory (see [4]), but for now this gets things working
>>>     without wasting too much memory.
>>>
>>> The ABI to the RMM from a realm (the RSI) is based on the final RMM v1.0
>>> (EAC 5) specification[2]. Future RMM specifications will be backwards
>>> compatible so a guest using the v1.0 specification (i.e. this series)
>>> will be able to run on future versions of the RMM without modification.
>>>
>>> This series is based on v6.10-rc1. It is also available as a git
>>> repository:
>>>
>>> https://gitlab.arm.com/linux-arm/linux-cca cca-guest/v4
>
> Which cca-host branch should I use for testing cca-guest/v4?
>
> I'm getting compilation errors with cca-host/v3 and cca-guest/v4, is there
> any known WAR or fix to resolve this issue?

cca-host/v3 should work with cca-guest/v4. I've been working on
rebasing/updating the branches and should be able to post v4/v5 series
next week.

>
> arch/arm64/kvm/rme.c: In function ‘kvm_realm_reset_id_aa64dfr0_el1’:
> ././include/linux/compiler_types.h:487:45: error: call to
> ‘__compiletime_assert_650’ declared with attribute error: FIELD_PREP:
> value too large for the field
>   487 |         _compiletime_assert(condition, msg,
> __compiletime_assert_, __COUNTER__)
>       |                                             ^
> ././include/linux/compiler_types.h:468:25: note: in definition of macro
> ‘__compiletime_assert’
>   468 |                         prefix ##
> suffix();                             \
>       |                         ^~~~~~
> ././include/linux/compiler_types.h:487:9: note: in expansion of macro
> ‘_compiletime_assert’
>   487 |         _compiletime_assert(condition, msg,
> __compiletime_assert_, __COUNTER__)
>       |         ^~~~~~~~~~~~~~~~~~~
> ./include/linux/build_bug.h:39:37: note: in expansion of macro
> ‘compiletime_assert’
>    39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond),
> msg)
>       |                                     ^~~~~~~~~~~~~~~~~~
> ./include/linux/bitfield.h:68:17: note: in expansion of macro
> ‘BUILD_BUG_ON_MSG’
>    68 |                 BUILD_BUG_ON_MSG(__builtin_constant_p(_val)
> ?           \
>       |                 ^~~~~~~~~~~~~~~~
> ./include/linux/bitfield.h:115:17: note: in expansion of macro
> ‘__BF_FIELD_CHECK’
>   115 |                 __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP:
> ");    \
>       |                 ^~~~~~~~~~~~~~~~
> arch/arm64/kvm/rme.c:315:16: note: in expansion of macro ‘FIELD_PREP’
>   315 |         val |= FIELD_PREP(ID_AA64DFR0_EL1_BRPs_MASK, bps - 1) |
>       |                ^~~~~~~~~~
> make[5]: *** [scripts/Makefile.build:244: arch/arm64/kvm/rme.o] Error 1
> make[4]: *** [scripts/Makefile.build:485: arch/arm64/kvm] Error 2
> make[3]: *** [scripts/Makefile.build:485: arch/arm64] Error 2
> make[3]: *** Waiting for unfinished jobs....
>
> I'm using gcc-13.3.0 compiler and cross-compiling on X86 machine.

I'm not sure quite how this happens. The 'value' (bps - 1) shouldn't be
considered constant, so I don't see how the compiler has decided to
complain here - the __builtin_constant_p() should really be evaluating to 0.

The only thing I can think of is if the compiler has somehow determined
that rmm_feat_reg0 is 0 - which in theory it could do if it knew that
kvm_init_rme() cannot succeed (rmi_features() would never be called, so
the variable will never be set). Which makes me wonder if you're
building with a PAGE_SIZE other than 4k?

Obviously the code should still build if that's the case (so this would
be a bug) but we don't currently support CCA with PAGE_SIZE != 4k.

Steve