Re: [PATCH] KVM: arm64: Add missing memory barriers when switching to pKVM's hyp pgd

From: Marc Zyngier <maz@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	oliver.upton@linux.dev, keirf@google.com,
	Quentin Perret <qperret@google.com>
Subject: Re: [PATCH] KVM: arm64: Add missing memory barriers when switching to pKVM's hyp pgd
Date: Thu, 04 Jan 2024 18:20:34 +0000	[thread overview]
Message-ID: <86frzd9czx.wl-maz@kernel.org> (raw)
In-Reply-To: <20240104164220.7968-1-will@kernel.org>

On Thu, 04 Jan 2024 16:42:20 +0000,
Will Deacon <will@kernel.org> wrote:
> 
> In commit f320bc742bc23 ("KVM: arm64: Prepare the creation of s1
> mappings at EL2"), pKVM switches from a temporary host-provided
> page-table to its own page-table at EL2. Since there is only a single
> TTBR for the nVHE hypervisor, this involves disabling and re-enabling
> the MMU in __pkvm_init_switch_pgd().
> 
> Unfortunately, the memory barriers here are not quite correct.
> Specifically:
> 
>   - A DSB is required to complete the TLB invalidation executed while
>     the MMU is disabled.
> 
>   - An ISB is required to make the new TTBR value visible to the
>     page-table walker before the MMU is enabled in the SCTLR.
> 
> An earlier version of the patch actually got this correct:
> 
>   https://lore.kernel.org/lkml/20210304184717.GB21795@willie-the-truck/
> 
> but thanks to some badly worded review comments from yours truly, these
> were dropped for the version that was eventually merged.
> 
> Bring back the barriers and fix the potential issue (but note that this
> was found by code inspection).
> 
> Cc: Quentin Perret <qperret@google.com>
> Fixes: f320bc742bc23 ("KVM: arm64: Prepare the creation of s1 mappings at EL2")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-init.S | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-init.S b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> index 1cc06e6797bd..9205e95f8529 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> @@ -292,6 +292,8 @@ alternative_else_nop_endif
>  	mov	sp, x0
>  
>  	/* And turn the MMU back on! */
> +	dsb	nsh
> +	isb
>  	set_sctlr_el2	x2
>  	ret	x1
>  SYM_FUNC_END(__pkvm_init_switch_pgd)

I'm happy to take the patch as is, but maybe we consider moving these
barriers into the set_sctlr helper as a future cleanup?

Thanks,

	M.

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

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