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Subject: Re: [PATCH v3 04/16] ioasid: Add custom IOASID allocator
To:     Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc:     iommu@lists.linux-foundation.org,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
        Yi Liu <yi.l.liu@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
References: <1556922737-76313-1-git-send-email-jacob.jun.pan@linux.intel.com>
 <1556922737-76313-5-git-send-email-jacob.jun.pan@linux.intel.com>
 <c991ff0c-87bf-574a-bf31-9b37f0421385@redhat.com>
 <20190522124255.117de6f6@jacob-builder>
From:   Auger Eric <eric.auger@redhat.com>
Message-ID: <a33797e9-d34b-b0a9-4f39-700dce8252b3@redhat.com>
Date:   Thu, 23 May 2019 09:14:07 +0200
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Hi Jacob,

On 5/22/19 9:42 PM, Jacob Pan wrote:
> On Tue, 21 May 2019 11:55:55 +0200
> Auger Eric <eric.auger@redhat.com> wrote:
> 
>> Hi Jacob,
>>
>> On 5/4/19 12:32 AM, Jacob Pan wrote:
>>> Sometimes, IOASID allocation must be handled by platform specific
>>> code. The use cases are guest vIOMMU and pvIOMMU where IOASIDs need
>>> to be allocated by the host via enlightened or paravirt interfaces.
>>>
>>> This patch adds an extension to the IOASID allocator APIs such that
>>> platform drivers can register a custom allocator, possibly at boot
>>> time, to take over the allocation. Xarray is still used for tracking
>>> and searching purposes internal to the IOASID code. Private data of
>>> an IOASID can also be set after the allocation.
>>>
>>> There can be multiple custom allocators registered but only one is
>>> used at a time. In case of hot removal of devices that provides the
>>> allocator, all IOASIDs must be freed prior to unregistering the
>>> allocator. Default XArray based allocator cannot be mixed with
>>> custom allocators, i.e. custom allocators will not be used if there
>>> are outstanding IOASIDs allocated by the default XA allocator.
>>>
>>> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
>>> ---
>>>  drivers/iommu/ioasid.c | 125
>>> +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed,
>>> 125 insertions(+)
>>>
>>> diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
>>> index 99f5e0a..ed2915a 100644
>>> --- a/drivers/iommu/ioasid.c
>>> +++ b/drivers/iommu/ioasid.c
>>> @@ -17,6 +17,100 @@ struct ioasid_data {
>>>  };
>>>  
>>>  static DEFINE_XARRAY_ALLOC(ioasid_xa);
>>> +static DEFINE_MUTEX(ioasid_allocator_lock);
>>> +static struct ioasid_allocator *active_custom_allocator;
>>> +
>>> +static LIST_HEAD(custom_allocators);
>>> +/*
>>> + * A flag to track if ioasid default allocator is in use, this will
>>> + * prevent custom allocator from being used. The reason is that
>>> custom allocator
>>> + * must have unadulterated space to track private data with
>>> xarray, there cannot
>>> + * be a mix been default and custom allocated IOASIDs.
>>> + */
>>> +static int default_allocator_active;
>>> +
>>> +/**
>>> + * ioasid_register_allocator - register a custom allocator
>>> + * @allocator: the custom allocator to be registered
>>> + *
>>> + * Custom allocators take precedence over the default xarray based
>>> allocator.
>>> + * Private data associated with the ASID are managed by ASID
>>> common code
>>> + * similar to data stored in xa.
>>> + *
>>> + * There can be multiple allocators registered but only one is
>>> active. In case
>>> + * of runtime removal of a custom allocator, the next one is
>>> activated based
>>> + * on the registration ordering.
>>> + */
>>> +int ioasid_register_allocator(struct ioasid_allocator *allocator)
>>> +{
>>> +	struct ioasid_allocator *pallocator;
>>> +	int ret = 0;
>>> +
>>> +	if (!allocator)
>>> +		return -EINVAL;  
>> is it really necessary? Sin't it the caller responsibility?
> makes sense. will remove this one and below.
>>> +
>>> +	mutex_lock(&ioasid_allocator_lock);
>>> +	/*
>>> +	 * No particular preference since all custom allocators
>>> end up calling
>>> +	 * the host to allocate IOASIDs. We activate the first one
>>> and keep
>>> +	 * the later registered allocators in a list in case the
>>> first one gets
>>> +	 * removed due to hotplug.
>>> +	 */
>>> +	if (list_empty(&custom_allocators))
>>> +		active_custom_allocator = allocator;> +
>>> else {
>>> +		/* Check if the allocator is already registered */
>>> +		list_for_each_entry(pallocator,
>>> &custom_allocators, list) {
>>> +			if (pallocator == allocator) {
>>> +				pr_err("IOASID allocator already
>>> registered\n");
>>> +				ret = -EEXIST;
>>> +				goto out_unlock;
>>> +			}
>>> +		}
>>> +	}
>>> +	list_add_tail(&allocator->list, &custom_allocators);
>>> +
>>> +out_unlock:
>>> +	mutex_unlock(&ioasid_allocator_lock);
>>> +	return ret;
>>> +}
>>> +EXPORT_SYMBOL_GPL(ioasid_register_allocator);
>>> +
>>> +/**
>>> + * ioasid_unregister_allocator - Remove a custom IOASID allocator
>>> + * @allocator: the custom allocator to be removed
>>> + *
>>> + * Remove an allocator from the list, activate the next allocator
>>> in
>>> + * the order it was registered.
>>> + */
>>> +void ioasid_unregister_allocator(struct ioasid_allocator
>>> *allocator) +{
>>> +	if (!allocator)
>>> +		return;  
>> is it really necessary?
>>> +
>>> +	if (list_empty(&custom_allocators)) {
>>> +		pr_warn("No custom IOASID allocators active!\n");
>>> +		return;
>>> +	}
>>> +
>>> +	mutex_lock(&ioasid_allocator_lock);
>>> +	list_del(&allocator->list);
>>> +	if (list_empty(&custom_allocators)) {
>>> +		pr_info("No custom IOASID allocators\n")>
>>> +		/*
>>> +		 * All IOASIDs should have been freed before the
>>> last custom
>>> +		 * allocator is unregistered. Unless default
>>> allocator is in
>>> +		 * use.
>>> +		 */
>>> +		BUG_ON(!xa_empty(&ioasid_xa)
>>> && !default_allocator_active);
>>> +		active_custom_allocator = NULL;
>>> +	} else if (allocator == active_custom_allocator) {  
>> In case you are removing the active custom allocator don't you also
>> need to check that all ioasids were freed. Otherwise you are likely
>> to switch to a different allocator whereas the asid space is
>> partially populated.
> The assumption is that all custom allocators on the same guest will end
> up calling the same host allocator. Having multiple custom allocators in
> the list is just a way to support multiple (p)vIOMMUs with hotplug.
> Therefore, we cannot nor need to free all PASIDs when one custom
> allocator goes away. This is a different situation then switching
> between default allocator and custom allocator, where custom allocator
> has to start with a clean space.
Although I understand your specific usecase, this framework may have
other users, where custom allocators behave differently.

Also the commit msg says:"
In case of hot removal of devices that provides the
allocator, all IOASIDs must be freed prior to unregistering the
allocator."

Thanks

Eric
> 
>  
>>> +		active_custom_allocator =
>>> list_entry(&custom_allocators, struct ioasid_allocator, list);
>>> +		pr_info("IOASID allocator changed");
>>> +	}
>>> +	mutex_unlock(&ioasid_allocator_lock);
>>> +}
>>> +EXPORT_SYMBOL_GPL(ioasid_unregister_allocator);
>>>  
>>>  /**
>>>   * ioasid_set_data - Set private data for an allocated ioasid
>>> @@ -68,6 +162,29 @@ ioasid_t ioasid_alloc(struct ioasid_set *set,
>>> ioasid_t min, ioasid_t max, data->set = set;
>>>  	data->private = private;
>>>  
>>> +	mutex_lock(&ioasid_allocator_lock);
>>> +	/*
>>> +	 * Use custom allocator if available, otherwise use
>>> default.
>>> +	 * However, if there are active IOASIDs already been
>>> allocated by default
>>> +	 * allocator, custom allocator cannot be used.
>>> +	 */
>>> +	if (!default_allocator_active && active_custom_allocator) {
>>> +		id = active_custom_allocator->alloc(min, max,
>>> active_custom_allocator->pdata);
>>> +		if (id == INVALID_IOASID) {
>>> +			pr_err("Failed ASID allocation by custom
>>> allocator\n");
>>> +			mutex_unlock(&ioasid_allocator_lock);
>>> +			goto exit_free;
>>> +		}
>>> +		/*
>>> +		 * Use XA to manage private data also sanitiy
>>> check custom
>>> +		 * allocator for duplicates.
>>> +		 */
>>> +		min = id;
>>> +		max = id + 1;
>>> +	} else
>>> +		default_allocator_active = 1;  
>> nit: true?
> yes, i can turn default_allocator_active into a bool type.
> 
>>> +	mutex_unlock(&ioasid_allocator_lock);
>>> +
>>>  	if (xa_alloc(&ioasid_xa, &id, data, XA_LIMIT(min, max),
>>> GFP_KERNEL)) { pr_err("Failed to alloc ioasid from %d to %d\n",
>>> min, max); goto exit_free;> @@ -91,9 +208,17 @@ void
>>> ioasid_free(ioasid_t ioasid) {
>>>  	struct ioasid_data *ioasid_data;
>>>  
>>> +	mutex_lock(&ioasid_allocator_lock);
>>> +	if (active_custom_allocator)
>>> +		active_custom_allocator->free(ioasid,
>>> active_custom_allocator->pdata);
>>> +	mutex_unlock(&ioasid_allocator_lock);
>>> +
>>>  	ioasid_data = xa_erase(&ioasid_xa, ioasid);
>>>  
>>>  	kfree_rcu(ioasid_data, rcu);
>>> +
>>> +	if (xa_empty(&ioasid_xa))
>>> +		default_allocator_active = 0;  
>> Isn't it racy? what if an xa_alloc occurs inbetween?
>>
>>
> yes, i will move it under the mutex. Thanks.
>>>  }
>>>  EXPORT_SYMBOL_GPL(ioasid_free);
>>>  
>>>   
>>
>> Thanks
>>
>> Eric
> 
> [Jacob Pan]
>