On Thu, Sep 30, 2021 at 08:49:03AM +0000, Tian, Kevin wrote: > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > Sent: Thursday, September 23, 2021 8:22 PM > > > > > > These are different things and need different bits. Since the ARM path > > > > has a lot more code supporting it, I'd suggest Intel should change > > > > their code to use IOMMU_BLOCK_NO_SNOOP and abandon > > IOMMU_CACHE. > > > > > > I didn't fully get this point. The end result is same, i.e. making the DMA > > > cache-coherent when IOMMU_CACHE is set. Or if you help define the > > > behavior of IOMMU_CACHE, what will you define now? > > > > It is clearly specifying how the kernel API works: > > > > !IOMMU_CACHE > > must call arch cache flushers > > IOMMU_CACHE - > > do not call arch cache flushers > > IOMMU_CACHE|IOMMU_BLOCK_NO_SNOOP - > > dot not arch cache flushers, and ignore the no snoop bit. > > Who will set IOMMU_BLOCK_NO_SNOOP? Basically only qemu due to specialized x86 hypervisor knowledge. The only purpose of this attribute is to support a specific virtualization use case where a whole bunch of stuff is broken together: - the cache maintenance instructions are not available to a guest - the guest isn't aware that the instructions don't work and tells the device to issue no-snoop TLPs - The device ignores the 'disable no-snoop' flag in the PCIe config space Thus things become broken. Jason
