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Message-ID: <fb2384a1-2036-0d67-9955-ed56b4bc4d13@huawei.com>
Date: Thu, 27 May 2021 19:16:55 +0800
From: Shenming Lu <lushenming@...wei.com>
To: Alex Williamson <alex.williamson@...hat.com>
CC: Cornelia Huck <cohuck@...hat.com>, Will Deacon <will@...nel.org>,
"Robin Murphy" <robin.murphy@....com>,
Joerg Roedel <joro@...tes.org>,
"Jean-Philippe Brucker" <jean-philippe@...aro.org>,
Eric Auger <eric.auger@...hat.com>, <kvm@...r.kernel.org>,
<linux-kernel@...r.kernel.org>,
<linux-arm-kernel@...ts.infradead.org>,
<iommu@...ts.linux-foundation.org>, <linux-api@...r.kernel.org>,
Kevin Tian <kevin.tian@...el.com>,
Lu Baolu <baolu.lu@...ux.intel.com>, <yi.l.liu@...el.com>,
Christoph Hellwig <hch@...radead.org>,
Jonathan Cameron <Jonathan.Cameron@...wei.com>,
"Barry Song" <song.bao.hua@...ilicon.com>,
<wanghaibin.wang@...wei.com>, <yuzenghui@...wei.com>
Subject: Re: [RFC PATCH v3 2/8] vfio/type1: Add a page fault handler
On 2021/5/25 6:11, Alex Williamson wrote:
> On Fri, 21 May 2021 14:38:52 +0800
> Shenming Lu <lushenming@...wei.com> wrote:
>
>> On 2021/5/19 2:58, Alex Williamson wrote:
>>> On Fri, 9 Apr 2021 11:44:14 +0800
>>> Shenming Lu <lushenming@...wei.com> wrote:
>>>
>>>> VFIO manages the DMA mapping itself. To support IOPF (on-demand paging)
>>>> for VFIO (IOMMU capable) devices, we add a VFIO page fault handler to
>>>> serve the reported page faults from the IOMMU driver.
>>>>
>>>> Signed-off-by: Shenming Lu <lushenming@...wei.com>
>>>> ---
>>>> drivers/vfio/vfio_iommu_type1.c | 114 ++++++++++++++++++++++++++++++++
>>>> 1 file changed, 114 insertions(+)
>>>>
>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>> index 45cbfd4879a5..ab0ff60ee207 100644
>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>> @@ -101,6 +101,7 @@ struct vfio_dma {
>>>> struct task_struct *task;
>>>> struct rb_root pfn_list; /* Ex-user pinned pfn list */
>>>> unsigned long *bitmap;
>>>> + unsigned long *iopf_mapped_bitmap;
>>>> };
>>>>
>>>> struct vfio_batch {
>>>> @@ -141,6 +142,16 @@ struct vfio_regions {
>>>> size_t len;
>>>> };
>>>>
>>>> +/* A global IOPF enabled group list */
>>>> +static struct rb_root iopf_group_list = RB_ROOT;
>>>> +static DEFINE_MUTEX(iopf_group_list_lock);
>>>> +
>>>> +struct vfio_iopf_group {
>>>> + struct rb_node node;
>>>> + struct iommu_group *iommu_group;
>>>> + struct vfio_iommu *iommu;
>>>> +};
>>>> +
>>>> #define IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu) \
>>>> (!list_empty(&iommu->domain_list))
>>>>
>>>> @@ -157,6 +168,10 @@ struct vfio_regions {
>>>> #define DIRTY_BITMAP_PAGES_MAX ((u64)INT_MAX)
>>>> #define DIRTY_BITMAP_SIZE_MAX DIRTY_BITMAP_BYTES(DIRTY_BITMAP_PAGES_MAX)
>>>>
>>>> +#define IOPF_MAPPED_BITMAP_GET(dma, i) \
>>>> + ((dma->iopf_mapped_bitmap[(i) / BITS_PER_LONG] \
>>>> + >> ((i) % BITS_PER_LONG)) & 0x1)
>>>
>>>
>>> Can't we just use test_bit()?
>>
>> Yeah, we can use it.
>>
>>>
>>>
>>>> +
>>>> #define WAITED 1
>>>>
>>>> static int put_pfn(unsigned long pfn, int prot);
>>>> @@ -416,6 +431,34 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
>>>> return ret;
>>>> }
>>>>
>>>> +/*
>>>> + * Helper functions for iopf_group_list
>>>> + */
>>>> +static struct vfio_iopf_group *
>>>> +vfio_find_iopf_group(struct iommu_group *iommu_group)
>>>> +{
>>>> + struct vfio_iopf_group *iopf_group;
>>>> + struct rb_node *node;
>>>> +
>>>> + mutex_lock(&iopf_group_list_lock);
>>>> +
>>>> + node = iopf_group_list.rb_node;
>>>> +
>>>> + while (node) {
>>>> + iopf_group = rb_entry(node, struct vfio_iopf_group, node);
>>>> +
>>>> + if (iommu_group < iopf_group->iommu_group)
>>>> + node = node->rb_left;
>>>> + else if (iommu_group > iopf_group->iommu_group)
>>>> + node = node->rb_right;
>>>> + else
>>>> + break;
>>>> + }
>>>> +
>>>> + mutex_unlock(&iopf_group_list_lock);
>>>> + return node ? iopf_group : NULL;
>>>> +}
>>>
>>> This looks like a pretty heavy weight operation per DMA fault.
>>>
>>> I'm also suspicious of this validity of this iopf_group after we've
>>> dropped the locking, the ordering of patches makes this very confusing.
>>
>> My thought was to include the handling of DMA faults completely in the type1
>> backend by introducing the vfio_iopf_group struct. But it seems that introducing
>> a struct with an unknown lifecycle causes more problems...
>> I will use the path from vfio-core as in the v2 for simplicity and validity.
>>
>> Sorry for the confusing, I will reconstruct the series later. :-)
>>
>>>
>>>> +
>>>> static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>>>> {
>>>> struct mm_struct *mm;
>>>> @@ -3106,6 +3149,77 @@ static int vfio_iommu_type1_dirty_pages(struct vfio_iommu *iommu,
>>>> return -EINVAL;
>>>> }
>>>>
>>>> +/* VFIO I/O Page Fault handler */
>>>> +static int vfio_iommu_type1_dma_map_iopf(struct iommu_fault *fault, void *data)
>>>
>>> >From the comment, this seems like the IOMMU fault handler (the
>>> construction of this series makes this difficult to follow) and
>>> eventually it handles more than DMA mapping, for example transferring
>>> faults to the device driver. "dma_map_iopf" seems like a poorly scoped
>>> name.
>>
>> Maybe just call it dev_fault_handler?
>
> Better.
>
>>>> +{
>>>> + struct device *dev = (struct device *)data;
>>>> + struct iommu_group *iommu_group;
>>>> + struct vfio_iopf_group *iopf_group;
>>>> + struct vfio_iommu *iommu;
>>>> + struct vfio_dma *dma;
>>>> + dma_addr_t iova = ALIGN_DOWN(fault->prm.addr, PAGE_SIZE);
>>>> + int access_flags = 0;
>>>> + unsigned long bit_offset, vaddr, pfn;
>>>> + int ret;
>>>> + enum iommu_page_response_code status = IOMMU_PAGE_RESP_INVALID;
>>>> + struct iommu_page_response resp = {0};
>>>> +
>>>> + if (fault->type != IOMMU_FAULT_PAGE_REQ)
>>>> + return -EOPNOTSUPP;
>>>> +
>>>> + iommu_group = iommu_group_get(dev);
>>>> + if (!iommu_group)
>>>> + return -ENODEV;
>>>> +
>>>> + iopf_group = vfio_find_iopf_group(iommu_group);
>>>> + iommu_group_put(iommu_group);
>>>> + if (!iopf_group)
>>>> + return -ENODEV;
>>>> +
>>>> + iommu = iopf_group->iommu;
>>>> +
>>>> + mutex_lock(&iommu->lock);
>>>
>>> Again, I'm dubious of our ability to grab this lock from an object with
>>> an unknown lifecycle and races we might have with that group being
>>> detached or DMA unmapped. Also, how effective is enabling IOMMU page
>>> faulting if we're serializing all faults within a container context?
>>
>> Did you mean "efficient"?
>
> Yes, that's more appropriate.
>
>> I also worry about this as the mapping and unmapping of the faulting pages
>> are all with the same lock...
>> Is there a way to parallel them? Or could we have more fine grained lock
>> control?
>
> It seems we need it; the current locking is designed for static
> mappings by the user, therefore concurrency hasn't been a priority.
I will try to implement it. :-)
> This again touches how far we want to extend type1 in the direction
> we intend to go with SVA/PASID support in IOASID. Thanks,
Reply in the cover.
Thanks,
Shenming
>
> Alex
>
> .
>
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