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

[Peter Xu]

On Thu, May 14, 2020 at 03:56:24PM -0700, Sean Christopherson wrote:
> 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.

What's the PTE overhead you mentioned?  We need to fill PTEs one by one on
fault even if the page is allocated in the kernel, am I right?

4K is only an example - we can also use more pages as the template.  However I
guess the kvm memslot count could be a limit..  Could I ask what's the normal
size of this 0xff region, and its distribution?

Thanks,

-- 
Peter Xu