linux-kernel - Re: [RFC] vfio/type1: handle case where IOMMU does not support PAGE

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Date:	Wed, 28 Oct 2015 19:00:25 +0100
From:	Eric Auger <eric.auger@...aro.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, eric.auger@...com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, suravee.suthikulpanit@....com,
	christoffer.dall@...aro.org, linux-kernel@...r.kernel.org,
	patches@...aro.org
Subject: Re: [RFC] vfio/type1: handle case where IOMMU does not support
 PAGE_SIZE size

On 10/28/2015 06:55 PM, Will Deacon wrote:
> On Wed, Oct 28, 2015 at 06:48:41PM +0100, Eric Auger wrote:
>> On 10/28/2015 06:37 PM, Alex Williamson wrote:
>>> Ok, so with hopefully correcting my understand of what this does, isn't
>>> this effectively the same:
>>>
>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>> index 57d8c37..7db4f5a 100644
>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>> @@ -403,13 +403,19 @@ static void vfio_remove_dma(struct vfio_iommu *iommu, stru
>>>  static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
>>>  {
>>>         struct vfio_domain *domain;
>>> -       unsigned long bitmap = PAGE_MASK;
>>> +       unsigned long bitmap = ULONG_MAX;
>>>  
>>>         mutex_lock(&iommu->lock);
>>>         list_for_each_entry(domain, &iommu->domain_list, next)
>>>                 bitmap &= domain->domain->ops->pgsize_bitmap;
>>>         mutex_unlock(&iommu->lock);
>>>  
>>> +       /* Some comment about how the IOMMU API splits requests */
>>> +       if (bitmap & ~PAGE_MASK) {
>>> +               bitmap &= PAGE_MASK;
>>> +               bitmap |= PAGE_SIZE;
>>> +       }
>>> +
>>>         return bitmap;
>>>  }
>> Yes, to me it is indeed the same
>>>  
>>> This would also expose to the user that we're accepting PAGE_SIZE, which
>>> we weren't before, so it was not quite right to just let them do it
>>> anyway.  I don't think we even need to get rid of the WARN_ONs, do we?
>>> Thanks,
>>
>> The end-user might be afraid of those latter. Personally I would get rid
>> of them but that's definitively up to you.
> 
> I think Alex's point is that the WARN_ON's won't trigger with this patch,
> because he clears those lower bits in the bitmap.
ah yes sure!

Thanks

Eric
> 
> Will
> 

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