Re: [PATCH] kvm: arm64: correctly align nVHE percpu data

From: David Brazdil <dbrazdil@google.com>
To: Jamie Iles <jamie@nuviainc.com>
Cc: Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] kvm: arm64: correctly align nVHE percpu data
Date: Fri, 13 Nov 2020 10:12:27 +0000	[thread overview]
Message-ID: <20201113101227.zb6ckyriobjrlkc5@google.com> (raw)
In-Reply-To: <20201113003143.3057-1-jamie@nuviainc.com>

Hi Jamie,

On Fri, Nov 13, 2020 at 12:31:43AM +0000, Jamie Iles wrote:
> The nVHE percpu data is partially linked but the nVHE linker script did
> not align the percpu section.  The PERCPU_INPUT macro would then align
> the data to a page boundary:
> 
>   #define PERCPU_INPUT(cacheline)					\
>   	__per_cpu_start = .;						\
>   	*(.data..percpu..first)						\
>   	. = ALIGN(PAGE_SIZE);						\
>   	*(.data..percpu..page_aligned)					\
>   	. = ALIGN(cacheline);						\
>   	*(.data..percpu..read_mostly)					\
>   	. = ALIGN(cacheline);						\
>   	*(.data..percpu)						\
>   	*(.data..percpu..shared_aligned)				\
>   	PERCPU_DECRYPTED_SECTION					\
>   	__per_cpu_end = .;
> 
> but then when the final vmlinux linking happens the hypervisor percpu
> data is included after page alignment and so the offsets potentially
> don't match.  This manifests as one of the CPUs getting lost when
> running kvm-unit-tests on EL2 entry and subsequent soft lockup.

You're right - the internal alignment of these subsections can be off after
linking into vmlinux. Could you just elaborate on how you managed to trigger
this? Which offsets don't match?

> 
> Fixes: 30c953911c43 ("kvm: arm64: Set up hyp percpu data for nVHE")
> Cc: David Brazdil <dbrazdil@google.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Signed-off-by: Jamie Iles <jamie@nuviainc.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp.lds.S | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp.lds.S b/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
> index bb2d986ff696..84edca959893 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
> @@ -13,6 +13,7 @@
>  
>  SECTIONS {
>  	HYP_SECTION(.text)
> +	. = ALIGN(PAGE_SIZE);

Could you add a comment so that we don't forget why it was needed?

Thanks,
David

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