linux-kernel - Re: [PATCH v10 05/11] vfio/pci: Register an iommu fault handler

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Date:   Fri, 13 Nov 2020 17:11:47 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Zenghui Yu <yuzenghui@...wei.com>, eric.auger.pro@...il.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu, joro@...tes.org,
        alex.williamson@...hat.com, jacob.jun.pan@...ux.intel.com,
        yi.l.liu@...el.com, robin.murphy@....com
Subject: Re: [PATCH v10 05/11] vfio/pci: Register an iommu fault handler

Hi Zenghui,

On 9/24/20 10:49 AM, Zenghui Yu wrote:
> Hi Eric,
> 
> On 2020/3/21 0:19, Eric Auger wrote:
>> Register an IOMMU fault handler which records faults in
>> the DMA FAULT region ring buffer. In a subsequent patch, we
>> will add the signaling of a specific eventfd to allow the
>> userspace to be notified whenever a new fault as shown up.
>>
>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> 
> [...]
> 
>> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
>> index 586b89debed5..69595c240baf 100644
>> --- a/drivers/vfio/pci/vfio_pci.c
>> +++ b/drivers/vfio/pci/vfio_pci.c
>> @@ -27,6 +27,7 @@
>>   #include <linux/vfio.h>
>>   #include <linux/vgaarb.h>
>>   #include <linux/nospec.h>
>> +#include <linux/circ_buf.h>
>>     #include "vfio_pci_private.h"
>>   @@ -283,6 +284,38 @@ static const struct vfio_pci_regops
>> vfio_pci_dma_fault_regops = {
>>       .add_capability = vfio_pci_dma_fault_add_capability,
>>   };
>>   +int vfio_pci_iommu_dev_fault_handler(struct iommu_fault *fault,
>> void *data)
>> +{
>> +    struct vfio_pci_device *vdev = (struct vfio_pci_device *)data;
>> +    struct vfio_region_dma_fault *reg =
>> +        (struct vfio_region_dma_fault *)vdev->fault_pages;
>> +    struct iommu_fault *new =
>> +        (struct iommu_fault *)(vdev->fault_pages + reg->offset +
>> +            reg->head * reg->entry_size);
> 
> Shouldn't 'reg->head' be protected under the fault_queue_lock? Otherwise
> things may change behind our backs...>
> We shouldn't take any assumption about how IOMMU driver would report the
> fault (serially or in parallel), I think.

Yes I modified the locking

Thanks

Eric
> 
>> +    int head, tail, size;
>> +    int ret = 0;
>> +
>> +    if (fault->type != IOMMU_FAULT_DMA_UNRECOV)
>> +        return -ENOENT;
>> +
>> +    mutex_lock(&vdev->fault_queue_lock);
>> +
>> +    head = reg->head;
>> +    tail = reg->tail;
>> +    size = reg->nb_entries;
>> +
>> +    if (CIRC_SPACE(head, tail, size) < 1) {
>> +        ret = -ENOSPC;
>> +        goto unlock;
>> +    }
>> +
>> +    *new = *fault;
>> +    reg->head = (head + 1) % size;
>> +unlock:
>> +    mutex_unlock(&vdev->fault_queue_lock);
>> +    return ret;
>> +}
>> +
>>   #define DMA_FAULT_RING_LENGTH 512
>>     static int vfio_pci_init_dma_fault_region(struct vfio_pci_device
>> *vdev)
>> @@ -317,6 +350,13 @@ static int vfio_pci_init_dma_fault_region(struct
>> vfio_pci_device *vdev)
>>       header->entry_size = sizeof(struct iommu_fault);
>>       header->nb_entries = DMA_FAULT_RING_LENGTH;
>>       header->offset = sizeof(struct vfio_region_dma_fault);
>> +
>> +    ret = iommu_register_device_fault_handler(&vdev->pdev->dev,
>> +                    vfio_pci_iommu_dev_fault_handler,
>> +                    vdev);
>> +    if (ret)
>> +        goto out;
>> +
>>       return 0;
>>   out:
>>       kfree(vdev->fault_pages);
> 
> 
> Thanks,
> Zenghui
>