Re: [PATCH v3 1/6] iommu/core: split mapping to page sizes assupported by the hardware

[Roedel, Joerg]

Hi Ohad,

On Mon, Oct 17, 2011 at 04:05:02AM -0400, Ohad Ben-Cohen wrote:

> > This whole thing is quite marginal though and also very easy to change
> > later, so we can start with the "driver-provided io_page_size"
> > approach for now.
> 
> Sorry, I just couldn't get myself to sign-off this as it really feels
> wrong to me.
> 
> This min_pagesz member is just cached by the core so it doesn't need to
> look it up every time we're mapping. Drivers shouldn't care about it, as it's
> completely internal to the iommu core. I'm afraid that pushing this to
> the drivers
> feels like redundant duplication of code and might also confuse developers.
> 
> Let me please suggest two alternatives:
> a) drop this min_pagesz cache completely. iommu core would then redundantly
>   re-calculate this every time something is mapped, but I hardly believe there
>   is going to be a measurable impact on performance.
> b) keep the current implementation for now, and fix this later (when we constify
>   struct iommu_ops *) by caching min_pagesz in a dynamically allocated iommu
>   context. Since this future "constify" patch will anyway need to change 'struct
>   bus_type', it would be a good opportunity to do this change at the same time.
> 
> I don't mind which of those approaches to take, and I also don't mind doing (b)
> myself later, in a separate patch. Your call.

I think option a) is the best. It should add only minimal overhead to
the iommu_map path.

> 
> >> This needs to be (left > 0). The drivers are allowed to unmap more then
> >> requested, so this value may turn negative.
> >
> > Good point. 'left' is size_t though, so i'll fix this a bit differently.
> 
> Fixed, please take a look:
> 
> >From 00b8b9373fe2d73da0280ac1e6ade4a701c95140 Mon Sep 17 00:00:00 2001
> From: Ohad Ben-Cohen <ohad@xxxxxxxxxx>
> Date: Mon, 10 Oct 2011 23:50:55 +0200
> Subject: [PATCH] iommu/core: split mapping to page sizes as supported
> by the hardware
> 
> When mapping a memory region, split it to page sizes as supported
> by the iommu hardware. Always prefer bigger pages, when possible,
> in order to reduce the TLB pressure.
> 
> The logic to do that is now added to the IOMMU core, so neither the iommu
> drivers themselves nor users of the IOMMU API have to duplicate it.
> 
> This allows a more lenient granularity of mappings; traditionally the
> IOMMU API took 'order' (of a page) as a mapping size, and directly let
> the low level iommu drivers handle the mapping, but now that the IOMMU
> core can split arbitrary memory regions into pages, we can remove this
> limitation, so users don't have to split those regions by themselves.
> 
> Currently the supported page sizes are advertised once and they then
> remain static. That works well for OMAP and MSM but it would probably
> not fly well with intel's hardware, where the page size capabilities
> seem to have the potential to be different between several DMA
> remapping devices.
> 
> register_iommu() currently sets a default pgsize behavior, so we can convert
> the IOMMU drivers in subsequent patches. After all the drivers
> are converted, the temporary default settings will be removed.
> 
> Mainline users of the IOMMU API (kvm and omap-iovmm) are adopted
> to deal with bytes instead of page order.
> 
> Many thanks to Joerg Roedel <Joerg.Roedel@xxxxxxx> for significant review!
> 
> Signed-off-by: Ohad Ben-Cohen <ohad@xxxxxxxxxx>
> Cc: David Brown <davidb@xxxxxxxxxxxxxx>
> Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Cc: Joerg Roedel <Joerg.Roedel@xxxxxxx>
> Cc: Stepan Moskovchenko <stepanm@xxxxxxxxxxxxxx>
> Cc: KyongHo Cho <pullip.cho@xxxxxxxxxxx>
> Cc: Hiroshi DOYU <hdoyu@xxxxxxxxxx>
> Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> ---
>  drivers/iommu/iommu.c      |  124 +++++++++++++++++++++++++++++++++++++++-----
>  drivers/iommu/omap-iovmm.c |   17 ++----
>  include/linux/iommu.h      |   24 +++++++-
>  virt/kvm/iommu.c           |    8 ++--
>  4 files changed, 141 insertions(+), 32 deletions(-)

The patch looks good now. Please implement option a) and it should be
fine. I will test it on an AMD IOMMU platform. We still need someone to
test it on VT-d.

	Joerg

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