lkml.org 
[lkml]   [2013]   [Oct]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v8 18/19] swiotlb-xen: no need to do execute two bus_to_phys translations in a row
On Wed, 23 Oct 2013, Konrad Rzeszutek Wilk wrote:
> On Wed, Oct 23, 2013 at 06:34:13PM +0100, Stefano Stabellini wrote:
> > > On Thu, Oct 17, 2013 at 06:43:33PM +0100, Stefano Stabellini wrote:
> > > > xen_unmap_single calls xen_bus_to_phys, then passes dev_addr to
> > > > is_xen_swiotlb_buffer that calls xen_bus_to_phys again.
> > >
> > > It does?
> > >
> > > I see it call mfn_to_local_pfn which calls pfn_to_mfn and
> > > get_phys_to_machine (which I presume is going to be a slow
> > > operation on ARM).
> >
> > Yeah, I am trying to reduce the number of pfn_to_mfn and mfn_to_pfn
> > translations.
> > That said, this patch is not needed to have a fully functional
> > swiotlb-xen on arm and I would OK with postponing it.
>
> OK, lets do that as it requires some hard thinking.

OK.


> > > I am bit uncomfortable with this - I recall when developing this
> > > that I had hit some odd cases of doing DMA of a page to a guest
> > > and it not always working. It was the case of not detecting whether
> > > the pfn was foreign or not. Sadly I can't recall the details.
> > >
> > > I would think that this problem would also be present with ARM?
> > > But perhaps all the new changes in the pfn_to_mfn that had lately
> > > been done have fixed this?
> >
> > No, because on ARM the guest is autotranslate so the pfn returned by
> > mfn_to_pfn is always "local".
> > However thanks to the m2p_override now we should get the "local" pfn
> > from mfn_to_pfn on x86 too.
> > I'll test this case though.
>
> Right, so maybe the pfn_to_mfn_local test is useless nowadays?

Yes, I think so. This might be better handled by removing
mfn_to_local_pfn altogether.


\
 
 \ /
  Last update: 2013-10-24 12:41    [W:1.793 / U:0.092 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site