linux-kernel - Re: [PATCH v2] vfio iommu: Add dma available capability

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Date:   Tue, 15 Sep 2020 09:22:13 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     alex.williamson@...hat.com, pmorel@...ux.ibm.com,
        schnelle@...ux.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vfio iommu: Add dma available capability

On 9/15/20 5:44 AM, Cornelia Huck wrote:
> On Mon, 14 Sep 2020 18:25:31 -0400
> Matthew Rosato <mjrosato@...ux.ibm.com> wrote:
> 
>> Commit 492855939bdb ("vfio/type1: Limit DMA mappings per container")
>> added the ability to limit the number of memory backed DMA mappings.
>> However on s390x, when lazy mapping is in use, we use a very large
>> number of concurrent mappings.  Let's provide the current allowable
>> number of DMA mappings to userspace via the IOMMU info chain so that
>> userspace can take appropriate mitigation.
>>
>> Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>
>> ---
>>   drivers/vfio/vfio_iommu_type1.c | 17 +++++++++++++++++
>>   include/uapi/linux/vfio.h       | 16 ++++++++++++++++
>>   2 files changed, 33 insertions(+)
> 
> (...)
> 
>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>> index 9204705..a8cc4a5 100644
>> --- a/include/uapi/linux/vfio.h
>> +++ b/include/uapi/linux/vfio.h
>> @@ -1039,6 +1039,22 @@ struct vfio_iommu_type1_info_cap_migration {
>>   	__u64	max_dirty_bitmap_size;		/* in bytes */
>>   };
>>   
>> +/*
>> + * The DMA available capability allows to report the current number of
>> + * simultaneously outstanding DMA mappings that are allowed.
>> + *
>> + * The structures below define version 1 of this capability.
> 
> "The structure below defines..." ?
> 

OK

>> + *
>> + * max: specifies the maximum number of outstanding DMA mappings allowed.
> 
> I think you forgot to tweak that one:
> 
> "avail: specifies the current number of outstanding DMA mappings allowed."
> 
> ?

Yep, that's a leftover from v1 :(

> 
>> + */
>> +#define VFIO_IOMMU_TYPE1_INFO_DMA_AVAIL 3
>> +
>> +struct vfio_iommu_type1_info_dma_avail {
>> +	struct	vfio_info_cap_header header;
>> +	__u32	avail;
>> +};
>> +
>> +
>>   #define VFIO_IOMMU_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 12)
>>   
>>   /**
>