Re: [Qemu-devel] [REBASED 0/2] exec: further refine address_space_get_iotlb_entry()

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 05, 2017 at 07:13:07PM +0200, Maxime Coquelin wrote:
> This series is a rebase of the first two patches of Peter's series
> improving address_space_get_iotlb_entry():
> Message-Id: <1496404254-17429-1-git-send-email-peterx@redhat.com>
> 
> It is actually not only an improvement, but fixes a regression in the way
> IOTLB updates sent to the backends are generated.
> The regression is introduced by patch:
> a764040cc8 ("exec: abstract address_space_do_translate()")
> 
> Prior to this patch IOTLB entries sent to the backend were aligned on the
> guest page boundaries (both addresses and size).
> For example, with the guest using 2MB pages:
>  * Backend sends IOTLB miss request for iova = 0x112378fb4
>  * QEMU replies with an IOTLB update with iova = 0x112200000, size = 0x200000
>  * Bakend insert above entry in its cache and compute the translation
> In this case, if the backend needs later to translate 0x112378004, it will
> result in a cache it and no need to send another IOTLB miss.
> 
> With this patch, the addr of the IOTLB entry will be the address requested
> via the IOTLB miss, the size is computed to cover the remaining of the guest
> page.
> The same example gives:
>  * Backend sends IOTLB miss request for iova = 0x112378fb4
>  * QEMU replies with an IOTLB update with iova = 112378fb4, size = 0x8704c
>  * Bakend insert above entry in its cache and compute the translation
> In this case, if the backend needs later to translate 0x112378004, it will
> result in another cache miss:
>  * Backend sends IOTLB miss request for iova = 0x112378004
>  * QEMU replies with an IOTLB update with iova = 0x112378004, size = 0x87FFC
>  * Bakend insert above entry in its cache and compute the translation
> It results in having much more IOTLB misses, and more importantly it pollutes
> the device IOTLB cache by multiplying the number of entries that moreover
> overlap.
> 
> Note that current Kernel & User backends implementation do not merge contiguous
> and overlapping IOTLB entries at device IOTLB cache insertion.
> 
> This series fixes this regression, so that IOTLB updates are aligned on
> guest's page boundaries.

Acked-by: Michael S. Tsirkin <mst@redhat.com>


> Peter Xu (2):
>   exec: add page_mask for flatview_do_translate
>   exec: simplify address_space_get_iotlb_entry
> 
>  exec.c | 75 +++++++++++++++++++++++++++++++++++++++++++-----------------------
>  1 file changed, 49 insertions(+), 26 deletions(-)
> 
> -- 
> 2.13.6