On 4/13/20 7:49 AM, Alex Williamson wrote: >> VFIO's unconditional page pinning is the real problem here IMNHO. They >> don't *really* need to pin the memory. We just don't have good >> paravirtualized IOMMU support or want to pay the runtime cost for >> pin/unpin operations. You *could* totally have speedy VM startup if >> only the pages being accessed or having DMA performed to them were >> allocated. But, the hacks that are in place mean that everything must >> be pinned. > Maybe in an SEV or Secure Boot environment we can assume the VM guest > OS uses the IOMMU exclusively for DMA, but otherwise the IOMMU is > optional (at least for x86, other archs do require IOMMU support > afaik). Therefore, how would we know which pages to pin when there are > only limited configs where we might be able to lean on the vIOMMU to > this extent? Thanks, You can delay pinning until the device is actually used. That should be late enough for the host to figure out whether a paravirtualized IOMMU is in place.
