Re: [PATCH v3 20/21] KVM: arm64: Remove unused 'pgd' field from 'struct kvm_s2_mmu'

From: Marc Zyngier <maz@kernel.org>
To: Gavin Shan <gshan@redhat.com>
Cc: kernel-team@android.com, Suzuki Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Quentin Perret <qperret@google.com>,
	James Morse <james.morse@arm.com>, Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 20/21] KVM: arm64: Remove unused 'pgd' field from 'struct kvm_s2_mmu'
Date: Fri, 04 Sep 2020 11:02:40 +0100	[thread overview]
Message-ID: <66b19fd347823e7111aafdb8d8251c14@kernel.org> (raw)
In-Reply-To: <7911796d-9124-cb7c-ace5-51e3b6d2dc28@redhat.com>

On 2020-09-04 01:59, Gavin Shan wrote:
> Hi Will,
> 
> On 9/4/20 2:50 AM, Will Deacon wrote:
>> On Thu, Sep 03, 2020 at 03:07:17PM +1000, Gavin Shan wrote:
>>> On 8/25/20 7:39 PM, Will Deacon wrote:
>>>> The stage-2 page-tables are entirely encapsulated by the 'pgt' field 
>>>> of
>>>> 'struct kvm_s2_mmu', so remove the unused 'pgd' field.
>>>> 
>>>> Cc: Marc Zyngier <maz@kernel.org>
>>>> Cc: Quentin Perret <qperret@google.com>
>>>> Signed-off-by: Will Deacon <will@kernel.org>
>>>> ---
>>>>    arch/arm64/include/asm/kvm_host.h | 1 -
>>>>    arch/arm64/kvm/mmu.c              | 2 --
>>>>    2 files changed, 3 deletions(-)
>>>> 
>>> 
>>> I think this might be folded into PATCH[18] as both patches are
>>> simple enough. I'm not sure the changes introduced in PATCH[19]
>>> prevent us doing this.
>>> 
>>> There is another question below.
>>> 
>>> Reviewed-by: Gavin Shan <gshan@redhat.com>
>>> 
>>>> diff --git a/arch/arm64/include/asm/kvm_host.h 
>>>> b/arch/arm64/include/asm/kvm_host.h
>>>> index 0b7c702b2151..41caf29bd93c 100644
>>>> --- a/arch/arm64/include/asm/kvm_host.h
>>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>>> @@ -79,7 +79,6 @@ struct kvm_s2_mmu {
>>>>    	 * for vEL1/EL0 with vHCR_EL2.VM == 0.  In that case, we use the
>>>>    	 * canonical stage-2 page tables.
>>>>    	 */
>>>> -	pgd_t		*pgd;
>>>>    	phys_addr_t	pgd_phys;
>>>>    	struct kvm_pgtable *pgt;
>>>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>>>> index ddeec0b03666..f28e03dcb897 100644
>>>> --- a/arch/arm64/kvm/mmu.c
>>>> +++ b/arch/arm64/kvm/mmu.c
>>>> @@ -384,7 +384,6 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct 
>>>> kvm_s2_mmu *mmu)
>>>>    	mmu->kvm = kvm;
>>>>    	mmu->pgt = pgt;
>>>>    	mmu->pgd_phys = __pa(pgt->pgd);
>>>> -	mmu->pgd = (void *)pgt->pgd;
>>>>    	mmu->vmid.vmid_gen = 0;
>>>>    	return 0;
>>>> @@ -470,7 +469,6 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>>>>    	spin_lock(&kvm->mmu_lock);
>>>>    	pgt = mmu->pgt;
>>>>    	if (pgt) {
>>>> -		mmu->pgd = NULL;
>>>>    		mmu->pgd_phys = 0;
>>>>    		mmu->pgt = NULL;
>>>>    		free_percpu(mmu->last_vcpu_ran);
>>>> 
>>> 
>>> I guess mmu->pgd_phys might be removed either because kvm_get_vttbr()
>>> is the only consumer.
>> 
>> Hmm, but kvm_get_vttbr() is still used after these patches, so I think
>> the pgd_phys field needs to stick around.
>> 
> 
> Yes, kvm_get_vttbr() is the only consumer. The corresponding physical
> address can be figured out in the function, we needn't have the 
> physical
> address and cache it in advance. However, it's not a big deal. I 
> probably
> post one patch to remove it after this series gets merged.
> 
>    baddr = __pa(mmu->pgt->pgd);

I'd rather you didn't. The NV patches need it for the AT emulation code,
so it might as well stay in place.

Thanks,

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

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