Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu walking

From: Marc Zyngier <maz@kernel.org>
To: Justin He <Justin.He@arm.com>
Cc: Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu, James Morse <James.Morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki Poulose <Suzuki.Poulose@arm.com>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Gavin Shan <gshan@redhat.com>,
	Yanan Wang <wangyanan55@huawei.com>,
	Quentin Perret <qperret@google.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu walking
Date: Thu, 04 Mar 2021 09:16:25 +0000	[thread overview]
Message-ID: <5bd4fa0b0590e548288b8f6dc3659ea5@kernel.org> (raw)
In-Reply-To: <AM6PR08MB437695640C272E4F065A8B2DF7979@AM6PR08MB4376.eurprd08.prod.outlook.com>

On 2021-03-04 00:46, Justin He wrote:
> Hi Marc
> 
>> -----Original Message-----
>> From: Will Deacon <will@kernel.org>
>> Sent: Thursday, March 4, 2021 5:13 AM
>> To: Marc Zyngier <maz@kernel.org>
>> Cc: Justin He <Justin.He@arm.com>; kvmarm@lists.cs.columbia.edu; James
>> Morse <James.Morse@arm.com>; Julien Thierry 
>> <julien.thierry.kdev@gmail.com>;
>> Suzuki Poulose <Suzuki.Poulose@arm.com>; Catalin Marinas
>> <Catalin.Marinas@arm.com>; Gavin Shan <gshan@redhat.com>; Yanan Wang
>> <wangyanan55@huawei.com>; Quentin Perret <qperret@google.com>; 
>> linux-arm-
>> kernel@lists.infradead.org; linux-kernel@vger.kernel.org
>> Subject: Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu 
>> walking
>> 
>> On Wed, Mar 03, 2021 at 07:07:37PM +0000, Marc Zyngier wrote:
>> > From e0524b41a71e0f17d6dc8f197e421e677d584e72 Mon Sep 17 00:00:00 2001
>> > From: Jia He <justin.he@arm.com>
>> > Date: Wed, 3 Mar 2021 10:42:25 +0800
>> > Subject: [PATCH] KVM: arm64: Fix range alignment when walking page tables
>> >
>> > When walking the page tables at a given level, and if the start
>> > address for the range isn't aligned for that level, we propagate
>> > the misalignment on each iteration at that level.
>> >
>> > This results in the walker ignoring a number of entries (depending
>> > on the original misalignment) on each subsequent iteration.
>> >
>> > Properly aligning the address at the before the next iteration
>> 
>> "at the before the next" ???
>> 
>> > addresses the issue.
>> >
>> > Cc: stable@vger.kernel.org
>> > Reported-by: Howard Zhang <Howard.Zhang@arm.com>
>> > Signed-off-by: Jia He <justin.he@arm.com>
>> > Fixes: b1e57de62cfb ("KVM: arm64: Add stand-alone page-table walker
>> infrastructure")
>> > [maz: rewrite commit message]
>> > Signed-off-by: Marc Zyngier <maz@kernel.org>
>> > Link: https://lore.kernel.org/r/20210303024225.2591-1-justin.he@arm.com
>> > ---
>> >  arch/arm64/kvm/hyp/pgtable.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
>> > index 4d177ce1d536..124cd2f93020 100644
>> > --- a/arch/arm64/kvm/hyp/pgtable.c
>> > +++ b/arch/arm64/kvm/hyp/pgtable.c
>> > @@ -223,7 +223,7 @@ static inline int __kvm_pgtable_visit(struct
>> kvm_pgtable_walk_data *data,
>> >  		goto out;
>> >
>> >  	if (!table) {
>> > -		data->addr += kvm_granule_size(level);
>> > +		data->addr = ALIGN(data->addr, kvm_granule_size(level));
> 
> What if previous data->addr is already aligned with 
> kvm_granule_size(level)?
> Hence a deadloop? Am I missing anything else?

Indeed, well spotted. I'll revert to your original suggestion
if everybody agrees...

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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