Re: [Xen-devel] Load increase after memory upgrade (part2)

[Carsten Schiers <carsten@xxxxxxxxxx>]

Should eventually mention that I create the DomU with only the parameter 
iommu=soft. I hope
Nothing more is required. For Xenified, it's swiotlb=32,force.

Carsten.

-----Ursprüngliche Nachricht-----
Von: Konrad Rzeszutek Wilk [mailto:konrad@xxxxxxxxxx] 
Gesendet: Freitag, 2. Dezember 2011 16:24
An: Konrad Rzeszutek Wilk
Cc: Ian Campbell; xen-devel; Carsten Schiers; zhenzhong.duan@xxxxxxxxxx; 
lersek@xxxxxxxxxx
Betreff: Re: [Xen-devel] Load increase after memory upgrade (part2)

> > > > > That is a puzzle. It should not. The code is very much the same - both
> > > > > use the generic SWIOTLB which has not changed for years.
> > > > 
> > > > The swiotlb-xen used by classic-xen kernels (which I assume is what
> > > > Carsten means by "Xenified") isn't exactly the same as the stuff in
> > > > mainline Linux, it's been heavily refactored for one thing. It's not
> > > > impossible that mainline is bouncing something it doesn't really need
> > > > to.
> > > 
> > > The usage, at least with 'pci_alloc_coherent' is that there is no bouncing
> > > being done. The alloc_coherent will allocate a nice page, underneath the 
> > > 4GB
> > > mark and give it to the driver. The driver can use it as it wishes and 
> > > there
> > > is no need to bounce buffer.
> > 
> > Oh, I didn't realise dma_alloc_coherent was part of swiotlb now. Only a
> > subset of swiotlb is in use then, all the bouncing stuff _should_ be
> > idle/unused -- but has that been confirmed?
> 
> Nope. I hope that the diagnostic patch I have in mind will prove/disprove 
> that.
> Now I just need to find a moment to write it :-)

Done!

Carsten, can you please patch your kernel with this hacky patch and
when you have booted the new kernel, just do

modprobe dump_swiotlb

it should give an idea of how many bounces are happening, coherent
allocations, syncs, and so on.. along with the last driver that
did those operations.



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