Re: [Qemu-devel] [PATCH 0/3] exec: further refine address_space_get_iotlb_entry()

[Michael S. Tsirkin]

On Fri, Jun 02, 2017 at 07:50:51PM +0800, Peter Xu wrote:
> With the patch applied:
> 
>   [PATCH v3] exec: fix address_space_get_iotlb_entry page mask
>   (already in Paolo's pull request but not yet merged)
> 
> Now we can have valid address masks. However it is still not ideal,
> considering that the mask may not be aligned to guest page sizes. One
> example would be when huge page is used in guest (please see commit
> message in patch 1 for details). It applies to normal pages too. So we
> not only need a valid address mask, we should make sure it is page
> mask (for x86, it should be either 4K/2M/1G pages).

Why should we? To get better performance, right?

> Patch 1+2 fixes the problem. Tested with both kernel net driver or
> testpmd, on either 4K/2M pages, to make sure the page mask is correct.
> 
> Patch 3 is cherry picked from PT series, after fixing from 1+2, we'll
> definitely want patch 3 now. Here's the simplest TCP streaming test
> using vhost dmar and iommu=pt in guest:
> 
>   without patch 3:    12.0Gbps

And what happens without patches 1-2?

>   with patch 3:       33.5Gbps

This is the part I don't get. Patches 1-2 will return a bigger region to
callers. The result should be better performance - instead it seems to
slow down vhost for some reason and we need tricks to get
performance back. What's going on?

> Please review, thanks.
> 
> Peter Xu (3):
>   exec: add page_mask for address_space_do_translate
>   exec: simplify address_space_get_iotlb_entry
>   vhost: iommu: cache static mapping if there is
> 
>  exec.c                 | 73 
> +++++++++++++++++++++++++++++++++-----------------
>  hw/virtio/trace-events |  4 +++
>  hw/virtio/vhost.c      | 66 +++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 119 insertions(+), 24 deletions(-)
> 
> -- 
> 2.7.4