linux-kernel - Re: [PATCH v4 01/10] KVM: arm64: Handle huge mappings for np-guest CMOs

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Message-ID: <86bjrsg3az.wl-maz@kernel.org>
Date: Fri, 16 May 2025 13:15:00 +0100
From: Marc Zyngier <maz@...nel.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: oliver.upton@...ux.dev,
	joey.gouly@....com,
	suzuki.poulose@....com,
	yuzenghui@...wei.com,
	catalin.marinas@....com,
	will@...nel.org,
	qperret@...gle.com,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	kernel-team@...roid.com
Subject: Re: [PATCH v4 01/10] KVM: arm64: Handle huge mappings for np-guest CMOs

On Fri, 09 May 2025 14:16:57 +0100,
Vincent Donnefort <vdonnefort@...gle.com> wrote:
> 
> clean_dcache_guest_page() and invalidate_icache_guest_page() accept a
> size as an argument. But they also rely on fixmap, which can only map a
> single PAGE_SIZE page.
> 
> With the upcoming stage-2 huge mappings for pKVM np-guests, those
> callbacks will get size > PAGE_SIZE. Loop the CMOs on a PAGE_SIZE basis
> until the whole range is done.
> 
> Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 31173c694695..23544928a637 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -219,14 +219,28 @@ static void guest_s2_put_page(void *addr)
>  
>  static void clean_dcache_guest_page(void *va, size_t size)
>  {
> -	__clean_dcache_guest_page(hyp_fixmap_map(__hyp_pa(va)), size);
> -	hyp_fixmap_unmap();
> +	WARN_ON(!PAGE_ALIGNED(size));

What if "va" isn't aligned?

> +
> +	while (size) {
> +		__clean_dcache_guest_page(hyp_fixmap_map(__hyp_pa(va)),
> +					  PAGE_SIZE);
> +		hyp_fixmap_unmap();
> +		va += PAGE_SIZE;
> +		size -= PAGE_SIZE;
> +	}

I know pKVM dies on WARN, but this code "looks" unsafe. Can you align
va and size to be on page boundaries, so that we are 100% sure the
loop terminates?

>  }
>  
>  static void invalidate_icache_guest_page(void *va, size_t size)
>  {
> -	__invalidate_icache_guest_page(hyp_fixmap_map(__hyp_pa(va)), size);
> -	hyp_fixmap_unmap();
> +	WARN_ON(!PAGE_ALIGNED(size));
> +
> +	while (size) {
> +		__invalidate_icache_guest_page(hyp_fixmap_map(__hyp_pa(va)),
> +					       PAGE_SIZE);
> +		hyp_fixmap_unmap();
> +		va += PAGE_SIZE;
> +		size -= PAGE_SIZE;
> +	}

Same here.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.