On Tue, Sep 2, 2014 at 5:25 PM, Benjamin Herrenschmidt <b...@au1.ibm.com> wrote: > On Tue, 2014-09-02 at 16:42 -0700, Andy Lutomirski wrote: >> So here's an ugly proposal: >> >> Step 1: Make virtio-pci use the DMA API only on x86. This will at >> least fix Xen and people experimenting with virtio hardware on x86, >> and it won't break anything, since there are no emulated IOMMUs on >> x86. > > I think we should make all virtio drivers use the DMA API and just have > different set of dma_ops. We can make a simple ifdef powerpc if needed > in virtio-pci that force the dma-ops of the device to some direct > "bypass" ops at init time. > > That way no need to select whether to use the DMA API or not, just > always use it, and add a tweak to replace the DMA ops with the direct > ones on the archs/platforms that need that. That was my original > proposal and I still think it's the best approach.
I agree *except* that implementing it will be a real PITA and (I think) can't be done without changing code in arch/. My patches plus an ifdef powerpc will be functionally equivalent, just uglier. > > As I said, make it always use the DMA API, but add a quirk to replace > the dma_ops with some NULL ops on platforms that need it. > > The only issue with that is the location of the dma ops is arch > specific, so that one function will contain some ifdefs, but the rest of > the code can just use the DMA API. Bigger quirk: on a standard s390 virtio guest configuration, dma_map_single etc will fail to link. I tried this in v1 of these patches. So we can poke at the archdata all day, but we can't build a kernel like that :( So until the dma_ops pointer move into struct device and CONFIG_HAS_DMA becomes mandatory (or mandatory enough that virtio can depend on it), I don't think we can do it this way. I'll send a v5 that is the same as v4 except with physical addressing hardcoded in for powerpc. --Andy _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
