Re: [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory

[Sean Christopherson]

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.