From: Andy Lutomirski <luto@kernel.org>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Peter Xu <peterx@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
kvm list <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Michael Tsirkin <mst@redhat.com>,
Julia Suvorova <jsuvorov@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, X86 ML <x86@kernel.org>
Subject: Re: [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory
Date: Thu, 14 May 2020 18:03:20 -0700 [thread overview]
Message-ID: <CALCETrUf3qMgpYGfF=b6dt2gneodTv5eYGGwKZ=xDSYDa9aTVg@mail.gmail.com> (raw)
In-Reply-To: <20200514225623.GF15847@linux.intel.com>
On Thu, May 14, 2020 at 3:56 PM Sean Christopherson
<sean.j.christopherson@intel.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?
next prev parent reply other threads:[~2020-05-15 1:03 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-14 18:05 [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 1/5] KVM: rename labels in kvm_init() Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 2/5] KVM: x86: introduce KVM_MEM_ALLONES memory Vitaly Kuznetsov
2020-05-14 19:18 ` Sean Christopherson
2020-05-15 8:24 ` Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 3/5] KVM: x86: move kvm_vcpu_gfn_to_memslot() out of try_async_pf() Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 4/5] KVM: x86: aggressively map PTEs in KVM_MEM_ALLONES slots Vitaly Kuznetsov
2020-05-14 19:46 ` Sean Christopherson
2020-05-15 8:36 ` Vitaly Kuznetsov
2020-05-15 13:58 ` Sean Christopherson
2020-05-14 18:05 ` [PATCH RFC 5/5] KVM: selftests: add KVM_MEM_ALLONES test Vitaly Kuznetsov
2020-05-14 22:05 ` [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory Peter Xu
2020-05-14 22:56 ` Sean Christopherson
2020-05-14 23:22 ` Peter Xu
2020-05-14 23:32 ` Sean Christopherson
2020-05-15 8:42 ` Vitaly Kuznetsov
2020-05-15 1:03 ` Andy Lutomirski [this message]
2020-05-15 11:15 ` Peter Xu
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