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From: Lu Baolu <baolu.lu@linux.intel.com>
To: Peter Xu <peterx@redhat.com>
Cc: kevin.tian@intel.com, Yi Sun <yi.y.sun@linux.intel.com>,
	ashok.raj@intel.com, kvm@vger.kernel.org,
	sanjay.k.kumar@intel.com, iommu@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org,
	Alex Williamson <alex.williamson@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	yi.y.sun@intel.com
Subject: Re: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces
Date: Tue, 8 Oct 2019 10:20:10 +0800
Message-ID: <27232499-4f13-83c0-a1d3-e82e9183f3f0@linux.intel.com> (raw)
In-Reply-To: <20190929052532.GA12953@xz-x1>

Hi,

On 9/29/19 1:25 PM, Peter Xu wrote:
> On Sat, Sep 28, 2019 at 04:23:16PM +0800, Lu Baolu wrote:
>> Hi Peter,
>>
>> On 9/27/19 1:34 PM, Peter Xu wrote:
>>> Hi, Baolu,
>>>
>>> On Fri, Sep 27, 2019 at 10:27:24AM +0800, Lu Baolu wrote:
>>>>>>>> +	spin_lock(&(domain)->page_table_lock);				\
>>>>>>>
>>>>>>> Is this intended to lock here instead of taking the lock during the
>>>>>>> whole page table walk?  Is it safe?
>>>>>>>
>>>>>>> Taking the example where nm==PTE: when we reach here how do we
>>>>>>> guarantee that the PMD page that has this PTE is still valid?
>>>>>>
>>>>>> We will always keep the non-leaf pages in the table,
>>>>>
>>>>> I see.  Though, could I ask why?  It seems to me that the existing 2nd
>>>>> level page table does not keep these when unmap, and it's not even use
>>>>> locking at all by leveraging cmpxchg()?
>>>>
>>>> I still need some time to understand how cmpxchg() solves the race issue
>>>> when reclaims pages. For example.
>>>>
>>>> Thread A				Thread B
>>>> -A1: check all PTE's empty		-B1: up-level PDE valid
>>>> -A2: clear the up-level PDE
>>>> -A3: reclaim the page			-B2: populate the PTEs
>>>>
>>>> Both (A1,A2) and (B1,B2) should be atomic. Otherwise, race could happen.
>>>
>>> I'm not sure of this, but IMHO it is similarly because we need to
>>> allocate the iova ranges from iova allocator first, so thread A (who's
>>> going to unmap pages) and thread B (who's going to map new pages)
>>> should never have collapsed regions if happening concurrently.  I'm
>>
>> Although they don't collapse, they might share a same pmd entry. If A
>> cleared the pmd entry and B goes ahead with populating the pte's. It
>> will crash.
> 
> My understanding is that if A was not owning all the pages on that PMD
> entry then it will never free the page that was backing that PMD
> entry.  Please refer to the code in dma_pte_clear_level() where it
> has:
> 
>          /* If range covers entire pagetable, free it */
>          if (start_pfn <= level_pfn &&
>                  last_pfn >= level_pfn + level_size(level) - 1) {
>                  ...
>          } else {
>                  ...
>          }
> 
> Note that when going into the else block, the PMD won't be freed but
> only the PTEs that upon the PMD will be cleared.

Exactly! Thanks for pointing this out.

I will do the same thing in v2.

> 
> In the case you mentioned above, IMHO it should go into that else
> block.  Say, thread A must not contain the whole range of that PMD
> otherwise thread B won't get allocated with pages within that range
> covered by the same PMD.
> 
> Thanks,
> 

Best regards,
Baolu
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Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-23 12:24 [RFC PATCH 0/4] Use 1st-level for DMA remapping in guest Lu Baolu
2019-09-23 12:24 ` [RFC PATCH 1/4] iommu/vt-d: Move domain_flush_cache helper into header Lu Baolu
2019-09-23 12:24 ` [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces Lu Baolu
2019-09-23 20:31   ` Raj, Ashok
2019-09-24  1:38     ` Lu Baolu
2019-09-25  4:30       ` Peter Xu
2019-09-25  4:38         ` Tian, Kevin
2019-09-25  5:24           ` Peter Xu
2019-09-25  6:52             ` Lu Baolu
2019-09-25  7:32               ` Tian, Kevin
2019-09-25  8:35                 ` Peter Xu
2019-09-26  1:42                 ` Lu Baolu
2019-09-25  5:21   ` Peter Xu
2019-09-26  2:35     ` Lu Baolu
2019-09-26  3:49       ` Peter Xu
2019-09-27  2:27         ` Lu Baolu
2019-09-27  5:34           ` Peter Xu
2019-09-28  8:23             ` Lu Baolu
2019-09-29  5:25               ` Peter Xu
2019-10-08  2:20                 ` Lu Baolu [this message]
2019-09-23 12:24 ` [RFC PATCH 3/4] iommu/vt-d: Map/unmap domain with mmmap/mmunmap Lu Baolu
2019-09-25  5:00   ` Tian, Kevin
2019-09-25  7:06     ` Lu Baolu
2019-09-23 12:24 ` [RFC PATCH 4/4] iommu/vt-d: Identify domains using first level page table Lu Baolu
2019-09-25  6:50   ` Peter Xu
2019-09-25  7:35     ` Tian, Kevin
2019-09-23 19:27 ` [RFC PATCH 0/4] Use 1st-level for DMA remapping in guest Jacob Pan
2019-09-23 20:25   ` Raj, Ashok
2019-09-24  4:40     ` Lu Baolu
2019-09-24  7:00     ` Tian, Kevin
2019-09-25  2:48       ` Lu Baolu
2019-09-25  6:56         ` Peter Xu
2019-09-25  7:21           ` Tian, Kevin
2019-09-25  7:45             ` Peter Xu
2019-09-25  8:02               ` Tian, Kevin
2019-09-25  8:52                 ` Peter Xu
2019-09-26  1:37                   ` Lu Baolu
2019-09-24  4:27   ` Lu Baolu

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