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

From: Yongji Xie
Date: Tue May 03 2016 - 02:03:13 EST

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.


+ 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;
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