linux-kernel - Re: [PATCH v8 29/43] arm64: RME: Always use 4k pages for realms

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Message-ID: <3103faef-1f02-47f9-b1ca-ec6af200773f@arm.com>
Date: Tue, 20 May 2025 15:59:57 +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>,
 Gavin Shan <gshan@...hat.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 Alper Gun <alpergun@...gle.com>, "Aneesh Kumar K . V"
 <aneesh.kumar@...nel.org>
Subject: Re: [PATCH v8 29/43] arm64: RME: Always use 4k pages for realms

On 16/04/2025 14:41, Steven Price wrote:
> Guest_memfd doesn't yet natively support huge pages, and there are
> currently difficulties for a VMM to manage huge pages efficiently so for
> now always split up mappings to PTE (4k).
> 
> The two issues that need progressing before supporting huge pages for
> realms are:
> 
>   1. guest_memfd needs to be able to allocate from an appropriate
>      allocator which can provide huge pages.
> 
>   2. The VMM needs to be able to repurpose private memory for a shared
>      mapping when the guest VM requests memory is transitioned. Because
>      this can happen at a 4k granularity it isn't possible to
>      free/reallocate while huge pages are in use. Allowing the VMM to
>      mmap() the shared portion of a huge page would allow the huge page
>      to be recreated when the memory is unshared and made protected again.
> 
> These two issues are not specific to realms and don't affect the realm
> API, so for now just break everything down to 4k pages in the RMM
> controlled stage 2. Future work can add huge page support without
> changing the uAPI.
> 
> Signed-off-by: Steven Price <steven.price@....com>

With comments from Gavin addressed,

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

> ---
> Changes since v7:
>   * Rewritten commit message
> ---
>   arch/arm64/kvm/mmu.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 02b66ee35426..29bab7a46033 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1653,6 +1653,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   	if (logging_active || is_protected_kvm_enabled()) {
>   		force_pte = true;
>   		vma_shift = PAGE_SHIFT;
> +	} else if (vcpu_is_rec(vcpu)) {
> +		// Force PTE level mappings for realms
> +		force_pte = true;
> +		vma_shift = PAGE_SHIFT;
>   	} else {
>   		vma_shift = get_vma_page_shift(vma, hva);
>   	}