Re: [Xen-devel] Xen ARM community call - meeting minutes and date for the next one

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 12 Jan 2017, Pooya.Keshavarzi wrote:
> > The other issue I heard about was some root file system corruptions after 
> > two or three re-boots we haven't observed in the native Linux case. The 
> > plan was to do some further analysis, first, before we blame Xen regarding 
> > this, though.
> > 
> > As mentioned, Pooya will have the details and correct me if I'm totally 
> > wrong here ;)
> > 
> 
> Firstly sorry for the late reply on this.
> 
> Regarding the problem with swiotlb-xen here are some more details:
> 
> If we limit Dom0's memory such that only low-memory (up to 32-bit addressable 
> memory) is available to Dom0, then swiotlb-xen does not have to use bounce 
> buffers and the devices (e.g. USB, ethernet) would work.
> 
> But when there is some high memory also available to Dom0, the followings 
> happen:
>  - If the the device address happens to be in the device's DMA window (see 
> xen_swiotlb_map_page()), then the device would work.
>  - Otherwise if it has to allocate and map a bounce buffer, then the device 
> would not work.

From what you wrote it looks like the xen_swiotlb_map_page path: 

        if (dma_capable(dev, dev_addr, size) &&
            !range_straddles_page_boundary(phys, size) &&
                !xen_arch_need_swiotlb(dev, phys, dev_addr) &&
                !swiotlb_force) {
                /* we are not interested in the dma_addr returned by
                 * xen_dma_map_page, only in the potential cache flushes 
executed
                 * by the function. */
                xen_dma_map_page(dev, page, dev_addr, offset, size, dir, attrs);
                return dev_addr;
        }

works, but the other does not. Does it match your understanding? Have
you done any digging to find the reason why the bounce buffer code path
is broken on your platform?

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