linux-kernel - Re: [PATCH v4 08/15] arm64: mm: Avoid TLBI when marking pages as valid

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Message-ID: <20240709115754.GD13242@willie-the-truck>
Date: Tue, 9 Jul 2024 12:57:54 +0100
From: Will Deacon <will@...nel.org>
To: Steven Price <steven.price@....com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	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 v4 08/15] arm64: mm: Avoid TLBI when marking pages as
 valid

On Mon, Jul 01, 2024 at 10:54:58AM +0100, Steven Price wrote:
> When __change_memory_common() is purely setting the valid bit on a PTE
> (e.g. via the set_memory_valid() call) there is no need for a TLBI as
> either the entry isn't changing (the valid bit was already set) or the
> entry was invalid and so should not have been cached in the TLB.
> 
> Signed-off-by: Steven Price <steven.price@....com>
> ---
> v4: New patch
> ---
>  arch/arm64/mm/pageattr.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 0e270a1c51e6..547a9e0b46c2 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -60,7 +60,13 @@ static int __change_memory_common(unsigned long start, unsigned long size,
>  	ret = apply_to_page_range(&init_mm, start, size, change_page_range,
>  					&data);
>  
> -	flush_tlb_kernel_range(start, start + size);
> +	/*
> +	 * If the memory is being made valid without changing any other bits
> +	 * then a TLBI isn't required as a non-valid entry cannot be cached in
> +	 * the TLB.
> +	 */
> +	if (pgprot_val(set_mask) != PTE_VALID || pgprot_val(clear_mask))
> +		flush_tlb_kernel_range(start, start + size);
>  	return ret;

Can you elaborate on when this actually happens, please? It feels like a
case of "Doctor, it hurts when I do this" rather than something we should
be trying to short-circuit in the low-level code.

Will