Re: [PATCH] vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio page is exclusive

[Yongji Xie <xyjxie@linux.vnet.ibm.com>]

On 2016/5/3 10:59, Tian, Kevin wrote:

>> From: Yongji Xie
>> Sent: Wednesday, April 27, 2016 8:22 PM
>>
>> Current vfio-pci implementation disallows to mmap
>> sub-page(size < PAGE_SIZE) MMIO BARs because these BARs' mmio
>> page may be shared with other BARs. This will cause some
>> performance issues when we passthrough a PCI device with
>> this kind of BARs. Guest will be not able to handle the mmio
>> accesses to the BARs which leads to mmio emulations in host.
>>
>> However, not all sub-page BARs will share page with other BARs.
>> We should allow to mmap those sub-page MMIO BARs which we can
>> make sure will not share page with other BARs.
>>
>> This patch adds support for this case. And we also try to use
>> shadow resource to reserve the remaind of the page which hot-add
>> device's BAR might be assigned into.
> 'shadow' usually means you have a corresponding part being
> shadowed, while here looks you mostly want some 'dummy'
> resource for reservation purpose?

Yes., 'dummy' may be better here. And I would also replace
shadow_res/shadow_resources_list with reserved_res/reserved_resources_list.

>> +
>> +		if (!(res->start & ~PAGE_MASK)) {
>> +			/*
>> +			 * Add shadow resource for sub-page bar whose mmio
>> +			 * page is exclusive in case that hot-add device's
>> +			 * bar is assigned into the mem hole.
>> +			 */
>> +			shadow_res = kzalloc(sizeof(*shadow_res), GFP_KERNEL);
>> +			shadow_res->resource.start = res->end + 1;
>> +			shadow_res->resource.end = res->start + PAGE_SIZE - 1;
> What about res->start not page aligned so you end up still having
> a portion before res->start not exclusively reserved?

Do you mean add a 'dummy' resource to reserve the portion
before res->start if res->start not page aligned?

But would it happen that there is a mem hole in the portion
before res->start? The resource should have been assigned
into the hole at the beginning.

Thanks,
Yongji

>> +			shadow_res->resource.flags = res->flags;
>> +			if (request_resource(res->parent,
>> +					&shadow_res->resource)) {
>> +				kfree(shadow_res);
>> +				return false;
>> +			}
>> +			shadow_res->index = index;
>> +			list_add(&shadow_res->res_next,
>> +					&vdev->shadow_resources_list);
>> +			return true;
> Thanks
> Kevin
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