Subject: Re: [PATCH] vfio/type1: Restore mapping performance with mdev support
To: Alex Williamson <alex.williamson@redhat.com>
References: <20161213205810.25950.32323.stgit@gimli.home>
 <2e2a2593-46ec-547b-e4a7-e78be446757a@nvidia.com>
 <20161215010347.3942360a@t450s.home>
 <02707161-145f-25f3-ab47-c63d1de81e02@nvidia.com>
 <20161215112748.10cbaa4c@t450s.home>
CC: <cjia@nvidia.com>, <linux-kernel@vger.kernel.org>,
        <kvm@vger.kernel.org>
From: Kirti Wankhede <kwankhede@nvidia.com>
Message-ID: <5f3a041a-6c48-ef1b-6bf4-43efc3c6303b@nvidia.com>
Date: Fri, 16 Dec 2016 23:54:24 +0530
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On 12/15/2016 11:57 PM, Alex Williamson wrote:
> On Thu, 15 Dec 2016 23:27:54 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 12/15/2016 1:33 PM, Alex Williamson wrote:
>>> On Thu, 15 Dec 2016 12:05:35 +0530
>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>   
>>>> On 12/14/2016 2:28 AM, Alex Williamson wrote:  
>>>>> As part of the mdev support, type1 now gets a task reference per
>>>>> vfio_dma and uses that to get an mm reference for the task while
>>>>> working on accounting.  That's the correct thing to do for paths
>>>>> where we can't rely on using current, but there are still hot paths
>>>>> where we can optimize because we know we're invoked by the user.
>>>>>
>>>>> Specifically, vfio_pin_pages_remote() is only called when the user
>>>>> does DMA mapping (vfio_dma_do_map) or if an IOMMU group is added to
>>>>> a container with existing mappings (vfio_iommu_replay).  We can
>>>>> therefore use current->mm as well as rlimit() and capable() directly
>>>>> rather than going through the high overhead path via the stored
>>>>> task_struct.  We also know that vfio_dma_do_unmap() is only called
>>>>> via user ioctl, so we can also tune that path to be more lightweight.
>>>>>
>>>>> In a synthetic guest mapping test emulating a 1TB VM backed by a
>>>>> single 4GB range remapped multiple times across the address space,
>>>>> the mdev changes to the type1 backend introduced a roughly 25% hit
>>>>> in runtime of this test.  These changes restore it to nearly the
>>>>> previous performance for the interfaces exercised here,
>>>>> VFIO_IOMMU_MAP_DMA and release on close.
>>>>>
>>>>> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
>>>>> ---
>>>>>  drivers/vfio/vfio_iommu_type1.c |  145 +++++++++++++++++++++------------------
>>>>>  1 file changed, 79 insertions(+), 66 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>>> index 9815e45..8dfeafb 100644
>>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>>> @@ -103,6 +103,10 @@ struct vfio_pfn {
>>>>>  #define IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu)	\
>>>>>  					(!list_empty(&iommu->domain_list))
>>>>>  
>>>>> +/* Make function bool options readable */
>>>>> +#define IS_CURRENT	(true)
>>>>> +#define DO_ACCOUNTING	(true)
>>>>> +
>>>>>  static int put_pfn(unsigned long pfn, int prot);
>>>>>  
>>>>>  /*
>>>>> @@ -264,7 +268,8 @@ static void vfio_lock_acct_bg(struct work_struct *work)
>>>>>  	kfree(vwork);
>>>>>  }
>>>>>  
>>>>> -static void vfio_lock_acct(struct task_struct *task, long npage)
>>>>> +static void vfio_lock_acct(struct task_struct *task,
>>>>> +			   long npage, bool is_current)
>>>>>  {
>>>>>  	struct vwork *vwork;
>>>>>  	struct mm_struct *mm;
>>>>> @@ -272,24 +277,31 @@ static void vfio_lock_acct(struct task_struct *task, long npage)
>>>>>  	if (!npage)
>>>>>  		return;
>>>>>  
>>>>> -	mm = get_task_mm(task);
>>>>> +	mm = is_current ? task->mm : get_task_mm(task);
>>>>>  	if (!mm)
>>>>> -		return; /* process exited or nothing to do */
>>>>> +		return; /* process exited */
>>>>>  
>>>>>  	if (down_write_trylock(&mm->mmap_sem)) {
>>>>>  		mm->locked_vm += npage;
>>>>>  		up_write(&mm->mmap_sem);
>>>>> -		mmput(mm);
>>>>> +		if (!is_current)
>>>>> +			mmput(mm);
>>>>>  		return;
>>>>>  	}
>>>>>  
>>>>> +	if (is_current) {
>>>>> +		mm = get_task_mm(task);
>>>>> +		if (!mm)
>>>>> +			return;
>>>>> +	}
>>>>> +
>>>>>  	/*
>>>>>  	 * Couldn't get mmap_sem lock, so must setup to update
>>>>>  	 * mm->locked_vm later. If locked_vm were atomic, we
>>>>>  	 * wouldn't need this silliness
>>>>>  	 */
>>>>>  	vwork = kmalloc(sizeof(struct vwork), GFP_KERNEL);
>>>>> -	if (!vwork) {
>>>>> +	if (WARN_ON(!vwork)) {
>>>>>  		mmput(mm);
>>>>>  		return;
>>>>>  	}
>>>>> @@ -345,13 +357,13 @@ static int put_pfn(unsigned long pfn, int prot)
>>>>>  }
>>>>>  
>>>>>  static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>>>>> -			 int prot, unsigned long *pfn)
>>>>> +			 int prot, unsigned long *pfn, bool is_current)
>>>>>  {
>>>>>  	struct page *page[1];
>>>>>  	struct vm_area_struct *vma;
>>>>>  	int ret;
>>>>>  
>>>>> -	if (mm == current->mm) {
>>>>> +	if (is_current) {    
>>>>
>>>> With this change, if vfio_pin_page_external() gets called from QEMU
>>>> process context, for example in response to some BAR0 register access,
>>>> it will still fallback to slow path, get_user_pages_remote(). We don't
>>>> have to change this function. This path already takes care of taking
>>>> best possible path.
>>>>
>>>> That also makes me think, vfio_pin_page_external() uses task structure
>>>> to get mlock limit and capability. Expectation is mdev vendor driver
>>>> shouldn't pin all system memory, but if any mdev driver does that, then
>>>> that driver might see such performance impact. Should we optimize this
>>>> path if (dma->task == current)?  
>>>
>>> Hi Kirti,
>>>
>>> I was actually trying to avoid the (task == current) test with this
>>> change because I wasn't sure how reliable it is.  Is there a
>>> possibility that this test generates a false positive if current
>>> coincidentally matches our task and does that allow us the same
>>> opportunities for making use of current that we have when we know in a
>>> process context execution path?  The above change makes this a more
>>> direct association.  Can you show that inferring the process context is
>>> correct?  Thanks,  
>>
>> We do hold the usage count of task structure, get_task_struct(current),
>> before saving its reference in dma->task which is released,
>> put_task_struct(), from vfio_remove_dma(). That makes sure that we have
>> a valid reference to task structure till we remove/free that dma
>> structure. Why would the check (dma->task == current) be false positive?
>> Vendor driver can call vfio_pin_pages() on access to some emulated
>> register from the same task who have mapped dma range, in that case this
>> check would be true.
> 
> I agree, the task matching dma->task cannot go away, the question is
> whether it's valid to use (current == dma->task) to detect if we're in
> the thread of execution of that task, or could we be detecting
> coincidental matches where we may get a different result if we're
> preempted by an interrupt.  TBH, I don't know how current works
> at that level of detail to know for sure, which is part of why I made
> the above change.  It seemed like the safer option to know for certain
> which path we're on rather than attempt to detect it.  Does current
> work as this requires, to detect that we're within the task's thread of
> execution and not simply temporarily aligned?  Thanks,
> 

If preempted by interrupt, then on restoring the task to run again, its
context should resume the current.
If your are still not comfortable, I'm OK to go ahead with your change
in vfio_pin_page_external(). But lets not change vaddr_get_pfn().

Thanks,
Kirti