Re: [PATCH v7 12/45] arm64: RME: Allocate/free RECs to match vCPUs

[Gavin Shan <gshan@redhat.com>]

On 2/14/25 2:13 AM, Steven Price wrote:
> The RMM maintains a data structure known as the Realm Execution Context
> (or REC). It is similar to struct kvm_vcpu and tracks the state of the
> virtual CPUs. KVM must delegate memory and request the structures are
> created when vCPUs are created, and suitably tear down on destruction.
> 
> RECs must also be supplied with addition pages - auxiliary (or AUX)
> granules - for storing the larger registers state (e.g. for SVE). The
> number of AUX granules for a REC depends on the parameters with which
> the Realm was created - the RMM makes this information available via the
> RMI_REC_AUX_COUNT call performed after creating the Realm Descriptor (RD).
> 
> Note that only some of register state for the REC can be set by KVM, the
> rest is defined by the RMM (zeroed). The register state then cannot be
> changed by KVM after the REC is created (except when the guest
> explicitly requests this e.g. by performing a PSCI call). The RMM also
> requires that the VMM creates RECs in ascending order of the MPIDR.
> 
> See Realm Management Monitor specification (DEN0137) for more information:
> https://developer.arm.com/documentation/den0137/
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Changes since v6:
>   * Avoid reporting the KVM_ARM_VCPU_REC feature if the guest isn't a
>     realm guest.
>   * Support host page size being larger than RMM's granule size when
>     allocating/freeing aux granules.
> Changes since v5:
>   * Separate the concept of vcpu_is_rec() and
>     kvm_arm_vcpu_rec_finalized() by using the KVM_ARM_VCPU_REC feature as
>     the indication that the VCPU is a REC.
> Changes since v2:
>   * Free rec->run earlier in kvm_destroy_realm() and adapt to previous patches.
> ---
>   arch/arm64/include/asm/kvm_emulate.h |   7 ++
>   arch/arm64/include/asm/kvm_host.h    |   3 +
>   arch/arm64/include/asm/kvm_rme.h     |  18 +++
>   arch/arm64/kvm/arm.c                 |  13 +-
>   arch/arm64/kvm/reset.c               |  11 ++
>   arch/arm64/kvm/rme.c                 | 179 +++++++++++++++++++++++++++
>   6 files changed, 229 insertions(+), 2 deletions(-)
> 

With the following one comment addressed:

Reviewed-by: Gavin Shan <gshan@redhat.com>

[...]

>   
>   /*
> diff --git a/arch/arm64/include/asm/kvm_rme.h b/arch/arm64/include/asm/kvm_rme.h
> index 698bb48a8ae1..5db377943db4 100644
> --- a/arch/arm64/include/asm/kvm_rme.h
> +++ b/arch/arm64/include/asm/kvm_rme.h
> @@ -6,6 +6,7 @@
>   #ifndef __ASM_KVM_RME_H
>   #define __ASM_KVM_RME_H
>   
> +#include <asm/rmi_smc.h>
>   #include <uapi/linux/kvm.h>
>   
>   /**
> @@ -65,6 +66,21 @@ struct realm {
>   	unsigned int ia_bits;
>   };
>   
> +/**
> + * struct realm_rec - Additional per VCPU data for a Realm
> + *
> + * @mpidr: MPIDR (Multiprocessor Affinity Register) value to identify this VCPU
> + * @rec_page: Kernel VA of the RMM's private page for this REC
> + * @aux_pages: Additional pages private to the RMM for this REC
> + * @run: Kernel VA of the RmiRecRun structure shared with the RMM
> + */
> +struct realm_rec {
> +	unsigned long mpidr;
> +	void *rec_page;
> +	struct page *aux_pages[REC_PARAMS_AUX_GRANULES];
> +	struct rec_run *run;
> +};
> +

REC_PARAMS_AUX_GRANULES represents the maximal number of the auxiliary granules.
Since the base page size is always larger than or equal to granule size (4KB).
The capacity of array @aux_pages[] needs to be REC_PARAMS_AUX_GRANULES. Ideally,
the array's size can be computed dynamically and it's allocated in kvm_create_rec().

Alternatively, to keep the code simple, a comment is needed here to explain why
the array's size has been set to REC_PARAMS_AUX_GRANULES.

An relevant question: Do we plan to support differentiated sizes between page
and granule? I had the assumption this feature will be supported in the future
after the base model (equal page and granule size) gets merged first.

Thanks,
Gavin