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Date:   Thu, 14 May 2020 18:03:20 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Peter Xu <peterx@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michael Tsirkin <mst@...hat.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory

On Thu, May 14, 2020 at 3:56 PM Sean Christopherson
<sean.j.christopherson@...el.com> 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.

How about a tiny character device driver /dev/ones?

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