linux-kernel - Re: [PATCH v2 12/43] arm64: RME: Keep a spare page delegated to the RMM

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Date: Wed, 17 Apr 2024 11:19:42 +0100
From: Suzuki K Poulose <suzuki.poulose@....com>
To: Steven Price <steven.price@....com>, kvm@...r.kernel.org,
 kvmarm@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
 Oliver Upton <oliver.upton@...ux.dev>, Zenghui Yu <yuzenghui@...wei.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Joey Gouly <joey.gouly@....com>, Alexandru Elisei
 <alexandru.elisei@....com>, Christoffer Dall <christoffer.dall@....com>,
 Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Subject: Re: [PATCH v2 12/43] arm64: RME: Keep a spare page delegated to the
 RMM

Hi Steven

On 12/04/2024 09:42, Steven Price wrote:
> Pages can only be populated/destroyed on the RMM at the 4KB granule,
> this requires creating the full depth of RTTs. However if the pages are
> going to be combined into a 4MB huge page the last RTT is only

minor nit: 2MB huge page.

> temporarily needed. Similarly when freeing memory the huge page must be
> temporarily split requiring temporary usage of the full depth oF RTTs.
> 
> To avoid needing to perform a temporary allocation and delegation of a
> page for this purpose we keep a spare delegated page around. In
> particular this avoids the need for memory allocation while destroying
> the realm guest.
> 
> Signed-off-by: Steven Price <steven.price@....com>
> ---
>   arch/arm64/include/asm/kvm_rme.h | 5 +++++
>   arch/arm64/kvm/rme.c             | 8 ++++++++
>   2 files changed, 13 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_rme.h b/arch/arm64/include/asm/kvm_rme.h
> index cf8cc4d30364..fba85e9ce3ae 100644
> --- a/arch/arm64/include/asm/kvm_rme.h
> +++ b/arch/arm64/include/asm/kvm_rme.h
> @@ -50,6 +50,9 @@ enum realm_state {
>    * @state: The lifetime state machine for the realm
>    * @rd: Kernel mapping of the Realm Descriptor (RD)
>    * @params: Parameters for the RMI_REALM_CREATE command
> + * @spare_page: A physical page that has been delegated to the Realm world but
> + *              is otherwise free. Used to avoid temporary allocation during
> + *              RTT operations.
>    * @num_aux: The number of auxiliary pages required by the RMM
>    * @vmid: VMID to be used by the RMM for the realm
>    * @ia_bits: Number of valid Input Address bits in the IPA
> @@ -60,6 +63,8 @@ struct realm {
>   	void *rd;
>   	struct realm_params *params;
>   
> +	phys_addr_t spare_page;
> +
>   	unsigned long num_aux;
>   	unsigned int vmid;
>   	unsigned int ia_bits;
> diff --git a/arch/arm64/kvm/rme.c b/arch/arm64/kvm/rme.c
> index 658d14e8d87d..9652ec6ab2fd 100644
> --- a/arch/arm64/kvm/rme.c
> +++ b/arch/arm64/kvm/rme.c
> @@ -103,6 +103,7 @@ static int realm_create_rd(struct kvm *kvm)
>   	}
>   
>   	realm->rd = rd;
> +	realm->spare_page = PHYS_ADDR_MAX;
>   
>   	if (WARN_ON(rmi_rec_aux_count(rd_phys, &realm->num_aux))) {
>   		WARN_ON(rmi_realm_destroy(rd_phys));
> @@ -283,6 +284,13 @@ void kvm_destroy_realm(struct kvm *kvm)
>   
>   	rme_vmid_release(realm->vmid);
>   
> +	if (realm->spare_page != PHYS_ADDR_MAX) {
> +		/* Leak the page if the undelegate fails */
> +		if (!WARN_ON(rmi_granule_undelegate(realm->spare_page)))
> +			free_page((unsigned long)phys_to_virt(realm->spare_page));
> +		realm->spare_page = PHYS_ADDR_MAX;
> +	}
> +
>   	for (i = 0; i < pgt->pgd_pages; i++) {
>   		phys_addr_t pgd_phys = kvm->arch.mmu.pgd_phys + i * PAGE_SIZE;
>   

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>