linux-kernel - RE: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

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Message-ID: <BN9PR11MB5433876DE6B6B92F9B50E3A58CCD9@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Wed, 1 Sep 2021 07:12:22 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Lutomirski, Andy" <luto@...nel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>
CC:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        "Dario Faggioli" <dfaggioli@...e.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: RE: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory

> From: Andy Lutomirski <luto@...nel.org>
> Sent: Wednesday, September 1, 2021 12:53 PM
> 
> On Thu, Aug 26, 2021, at 7:31 PM, Yu Zhang wrote:
> > On Thu, Aug 26, 2021 at 12:15:48PM +0200, David Hildenbrand wrote:
> 
> > Thanks a lot for this summary. A question about the requirement: do we or
> > do we not have plan to support assigned device to the protected VM?
> >
> > If yes. The fd based solution may need change the VFIO interface as well(
> > though the fake swap entry solution need mess with VFIO too). Because:
> >
> > 1> KVM uses VFIO when assigning devices into a VM.
> >
> > 2> Not knowing which GPA ranges may be used by the VM as DMA buffer,
> all
> > guest pages will have to be mapped in host IOMMU page table to host
> pages,
> > which are pinned during the whole life cycle fo the VM.
> >
> > 3> IOMMU mapping is done during VM creation time by VFIO and IOMMU
> driver,
> > in vfio_dma_do_map().
> >
> > 4> However, vfio_dma_do_map() needs the HVA to perform a GUP to get
> the HPA
> > and pin the page.
> >
> > But if we are using fd based solution, not every GPA can have a HVA, thus
> > the current VFIO interface to map and pin the GPA(IOVA) wont work. And I
> > doubt if VFIO can be modified to support this easily.
> >
> >
> 
> Do you mean assigning a normal device to a protected VM or a hypothetical
> protected-MMIO device?
> 
> If the former, it should work more or less like with a non-protected VM.
> mmap the VFIO device, set up a memslot, and use it.  I'm not sure whether
> anyone will actually do this, but it should be possible, at least in principle.
> Maybe someone will want to assign a NIC to a TDX guest.  An NVMe device
> with the understanding that the guest can't trust it wouldn't be entirely crazy
> ether.
> 
> If the latter, AFAIK there is no spec for how it would work even in principle.
> Presumably it wouldn't work quite like VFIO -- instead, the kernel could have
> a protection-virtual-io-fd mechanism, and that fd could be bound to a
> memslot in whatever way we settle on for binding secure memory to a
> memslot.

FYI the iommu logic in VFIO is being refactored out into an unified /dev/iommu
framework [1]. Currently it plans to support the same DMA mapping semantics
as what VFIO provides today (HVA-based). in the future it could be extended
to support another mapping protocol which accepts fd+offset instead of HVA 
and then calls the helper function from whatever backing store which can help 
translate fd+offset to HPA instead of using GUP.

Thanks
Kevin

[1]https://lore.kernel.org/kvm/BN9PR11MB5433B1E4AE5B0480369F97178C189@BN9PR11MB5433.namprd11.prod.outlook.com/