linux-kernel - Re: [PATCH 1/2] vfio/type1: Adopt fast IOTLB flush interface when unmap IOVAs

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Date:   Mon, 27 Nov 2017 15:12:25 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        joro@...tes.org, jroedel@...e.de
Subject: Re: [PATCH 1/2] vfio/type1: Adopt fast IOTLB flush interface when
 unmap IOVAs

Hi Alex,

On 11/18/17 4:51 AM, Alex Williamson wrote:
> On Fri, 17 Nov 2017 15:11:19 -0600
> Suravee Suthikulpanit<Suravee.Suthikulpanit@....com>  wrote:
> 
>> From: Suravee Suthikulpanit<suravee.suthikulpanit@....com>
>>
>> VFIO IOMMU type1 currently upmaps IOVA pages synchronously, which requires
>> IOTLB flushing for every unmapping. This results in large IOTLB flushing
>> overhead when handling pass-through devices with a large number of mapped
>> IOVAs (e.g. GPUs).
> Of course the type of device is really irrelevant, QEMU maps the entire
> VM address space for any assigned device.

Right, in this case certain high performance dGPUs map large address space.

> 
>> This can be avoided by using the new IOTLB flushing interface.
>>
>> Cc: Alex Williamson<alex.williamson@...hat.com>
>> Cc: Joerg Roedel<jroedel@...e.de>
>> Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@....com>
>> ---
>>   drivers/vfio/vfio_iommu_type1.c | 12 +++++++++---
>>   1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 92155cc..28a7ab6 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -698,10 +698,12 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>>   				break;
>>   		}
>>   
>> -		unmapped = iommu_unmap(domain->domain, iova, len);
>> +		unmapped = iommu_unmap_fast(domain->domain, iova, len);
>>   		if (WARN_ON(!unmapped))
>>   			break;
>>   
>> +		iommu_tlb_range_add(domain->domain, iova, len);
>> +
> We should only add @unmapped, not @len, right?

Hm right, I will change this in V2.

Thanks,
Suravee