On Thu, May 14, 2020 at 08:05:35PM +0200, Vitaly Kuznetsov wrote: > The idea of the patchset was suggested by Michael S. Tsirkin. > > PCIe config space can (depending on the configuration) be quite big but > usually is sparsely populated. Guest may scan it by accessing individual > device's page which, when device is missing, is supposed to have 'pci > holes' semantics: reads return '0xff' and writes get discarded. Currently, > userspace has to allocate real memory for these holes and fill them with > '0xff'. Moreover, different VMs usually require different memory. > > The idea behind the feature introduced by this patch is: let's have a > single read-only page filled with '0xff' in KVM and map it to all such > PCI holes in all VMs. This will free userspace of obligation to allocate > real memory and also allow us to speed up access to these holes as we > can aggressively map the whole slot upon first fault. > > RFC. I've only tested the feature with the selftest (PATCH5) on Intel/AMD > with and wiuthout EPT/NPT. I haven't tested memslot modifications yet. > > Patches are against kvm/next. Hi, Vitaly, Could this be done in userspace with existing techniques? E.g., shm_open() with a handle and fill one 0xff page, then remap it to anywhere needed in QEMU? Thanks, -- Peter Xu
