On Thu, May 14, 2020 at 06:05:16PM -0400, Peter Xu wrote: > 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?
Mapping that 4k page over and over is going to get expensive, e.g. each duplicate will need a VMA and a memslot, plus any PTE overhead. If the total sum of the holes is >2mb it'll even overflow the mumber of allowed memslots.
