Re: [PATCH] iommu/iova: Separate out rcache init

From: John Garry <john.garry@huawei.com>
To: Robin Murphy <robin.murphy@arm.com>,
	"joro@8bytes.org" <joro@8bytes.org>,
	"will@kernel.org" <will@kernel.org>,
	"mst@redhat.com" <mst@redhat.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>
Cc: "xieyongji@bytedance.com" <xieyongji@bytedance.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	Linuxarm <linuxarm@huawei.com>
Subject: Re: [PATCH] iommu/iova: Separate out rcache init
Date: Wed, 26 Jan 2022 17:58:04 +0000	[thread overview]
Message-ID: <0ec8829e-aef3-eee7-17cf-416b28db3c4c@huawei.com> (raw)
In-Reply-To: <ee4593b8-cdf6-935a-0eaf-48a8bfeae912@arm.com>

Hi Robin,

>>
>> Signed-off-by: John Garry<john.garry@huawei.com>
> Mangled patch? (no "---" separator here)

hmm... not sure. As an experiment, I just downloaded this patch from 
lore.kernel.org and it applies ok.

> 
> Overall this looks great, just a few comments further down...
> 

...

>> +}
>> +EXPORT_SYMBOL_GPL(iova_domain_init_rcaches);
>> +
>> +void iova_domain_free_rcaches(struct iova_domain *iovad)
>> +{
>> +	cpuhp_state_remove_instance_nocalls(CPUHP_IOMMU_IOVA_DEAD,
>> +					    &iovad->cpuhp_dead);
>> +	free_iova_rcaches(iovad);
>>    }
>> +EXPORT_SYMBOL_GPL(iova_domain_free_rcaches);
> I think we should continue to expect external callers to clean up with
> put_iova_domain(). 

ok, fine, makes sense

> If they aren't doing that already they have a bug
> (albeit minor), and we don't want to give the impression that it's OK to
> free the caches at any point*other*  than tearing down the whole
> iova_domain, since the implementation really wouldn't expect that.
> 
>>    /*
>>     * Try inserting IOVA range starting with 'iova_pfn' into 'rcache', and
>> @@ -831,7 +872,7 @@ static unsigned long iova_rcache_get(struct iova_domain *iovad,
>>    {
>>    	unsigned int log_size = order_base_2(size);
>>    
>> -	if (log_size >= IOVA_RANGE_CACHE_MAX_SIZE)
>> +	if (log_size >= IOVA_RANGE_CACHE_MAX_SIZE || !iovad->rcaches)
>>    		return 0;
>>    

..

>> @@ -102,6 +92,8 @@ struct iova *reserve_iova(struct iova_domain *iovad, unsigned long pfn_lo,
>>    	unsigned long pfn_hi);
>>    void init_iova_domain(struct iova_domain *iovad, unsigned long granule,
>>    	unsigned long start_pfn);
>> +int iova_domain_init_rcaches(struct iova_domain *iovad);
>> +void iova_domain_free_rcaches(struct iova_domain *iovad);
> As above, I vote for just forward-declaring the free routine in iova.c
> and keeping it entirely private.

ok

> 
>>    struct iova *find_iova(struct iova_domain *iovad, unsigned long pfn);
>>    void put_iova_domain(struct iova_domain *iovad);
>>    #else
>> @@ -157,6 +149,15 @@ static inline void init_iova_domain(struct iova_domain *iovad,
>>    {
>>    }
>>    
>> +static inline int iova_domain_init_rcaches(struct iova_domain *iovad)
>> +{
>> +	return -ENOTSUPP;
>> +}
>> +
>> +static inline void iova_domain_free_rcaches(struct iova_domain *iovad)
>> +{
>> +}
>> +
> I'd be inclined not to add stubs at all - I think it's a reasonable
> assumption that anyone involved enough to care about rcaches has a hard
> dependency on IOMMU_IOVA already.

So iova_domain_free_rcaches() would disappear from here as a result of 
the changes discussed above.

As for iova_domain_init_rcaches(), I was just following the IOMMU/IOVA 
coding practice - that is, always stub.

> It's certainly the case today, and I'd
> hardly want to encourage more users anyway.

I think that stronger deterrents would be needed :)

Anyway, I can remove it.

Thanks,
John