From: Yu Zhang <yu.c.zhang@linux.intel.com>
To: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>,
Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm list <kvm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Borislav Petkov <bp@alien8.de>,
Andrew Morton <akpm@linux-foundation.org>,
Joerg Roedel <jroedel@suse.de>, Andi Kleen <ak@linux.intel.com>,
David Rientjes <rientjes@google.com>,
Vlastimil Babka <vbabka@suse.cz>,
Tom Lendacky <thomas.lendacky@amd.com>,
Thomas Gleixner <tglx@linutronix.de>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Varad Gautam <varad.gautam@suse.com>,
Dario Faggioli <dfaggioli@suse.com>,
the arch/x86 maintainers <x86@kernel.org>,
linux-mm@kvack.org, linux-coco@lists.linux.dev,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
Sathyanarayanan Kuppuswamy
<sathyanarayanan.kuppuswamy@linux.intel.com>,
Dave Hansen <dave.hansen@intel.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory
Date: Thu, 2 Sep 2021 16:34:53 +0800 [thread overview]
Message-ID: <20210902083453.aeouc6fob53ydtc2@linux.intel.com> (raw)
In-Reply-To: <2b2740ec-fa89-e4c3-d175-824e439874a6@redhat.com>
On Wed, Sep 01, 2021 at 06:27:20PM +0200, David Hildenbrand wrote:
> On 01.09.21 18:07, Andy Lutomirski wrote:
> > On 9/1/21 3:24 AM, Yu Zhang wrote:
> > > On Tue, Aug 31, 2021 at 09:53:27PM -0700, Andy Lutomirski wrote:
> > > >
> > > >
> > > > 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.
> > > >
> > >
> > > Thanks Andy. I was asking the first scenario.
> > >
> > > Well, I agree it is doable if someone really want some assigned
> > > device in TD guest. As Kevin mentioned in his reply, HPA can be
> > > generated, by extending VFIO with a new mapping protocol which
> > > uses fd+offset, instead of HVA.
> >
> > I'm confused. I don't see why any new code is needed for this at all.
> > Every proposal I've seen for handling TDX memory continues to handle TDX
> > *shared* memory exactly like regular guest memory today. The only
> > differences are that more hole punching will be needed, which will
> > require lightweight memslots (to have many of them), memslots with
> > holes, or mappings backing memslots with holes (which can be done with
> > munmap() on current kernels).
> >
> > So you can literally just mmap a VFIO device and expect it to work,
> > exactly like it does right now. Whether the guest will be willing to
> > use the device will depend on the guest security policy (all kinds of
> > patches about that are flying around), but if the guest tries to use it,
> > it really should just work.
>
> ... but if you end up mapping private memory into IOMMU of the device and
> the device ends up accessing that memory, we're in the same position that
> the host might get capped, just like access from user space, no?
Well, according to the spec:
- If the result of the translation results in a physical address with a TD
private key ID, then the IOMMU will abort the transaction and report a VT-d
DMA remapping failure.
- If the GPA in the transaction that is input to the IOMMU is private (SHARED
bit is 0), then the IOMMU may abort the transaction and report a VT-d DMA
remapping failure.
So I guess mapping private GPAs in IOMMU is not that dangerous as mapping
into userspace. Though still wrong.
>
> Sure, you can map only the complete duplicate shared-memory region into the
> IOMMU of the device, that would work. Shame vfio mostly always pins all
> guest memory and you essentially cannot punch holes into the shared memory
> anymore -- resulting in the worst case in a duplicate memory consumption for
> your VM.
>
> So you'd actually want to map only the *currently* shared pieces into the
> IOMMU and update the mappings on demand. Having worked on something related,
Exactly. On demand mapping and page pinning for shared memory is necessary.
> I can only say that 64k individual mappings, and not being able to modify
> existing mappings except completely deleting them to replace with something
> new (!atomic), can be quite an issue for bigger VMs.
Do you mean atomicity in mapping/unmapping can hardly be guaranteed during
the shared <-> private transition? May I ask for elaboration? Thanks!
B.R.
Yu
next prev parent reply other threads:[~2021-09-02 8:35 UTC|newest]
Thread overview: 71+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-24 0:52 [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory Sean Christopherson
2021-08-24 10:48 ` Yu Zhang
2021-08-26 0:35 ` Sean Christopherson
2021-08-26 13:23 ` Yu Zhang
2021-08-26 10:15 ` David Hildenbrand
2021-08-26 17:05 ` Andy Lutomirski
2021-08-26 21:26 ` David Hildenbrand
2021-08-27 18:24 ` Andy Lutomirski
2021-08-27 22:28 ` Sean Christopherson
2021-08-31 19:12 ` David Hildenbrand
2021-08-31 20:45 ` Sean Christopherson
2021-09-01 7:51 ` David Hildenbrand
2021-08-27 2:31 ` Yu Zhang
2021-08-31 19:08 ` David Hildenbrand
2021-08-31 20:01 ` Andi Kleen
2021-08-31 20:15 ` David Hildenbrand
2021-08-31 20:39 ` Andi Kleen
2021-09-01 3:34 ` Yu Zhang
2021-09-01 4:53 ` Andy Lutomirski
2021-09-01 7:12 ` Tian, Kevin
2021-09-01 10:24 ` Yu Zhang
2021-09-01 16:07 ` Andy Lutomirski
2021-09-01 16:27 ` David Hildenbrand
2021-09-02 8:34 ` Yu Zhang [this message]
2021-09-02 8:44 ` David Hildenbrand
2021-09-02 11:02 ` Yu Zhang
2021-09-02 8:19 ` Yu Zhang
2021-09-02 18:41 ` Andy Lutomirski
2021-09-07 1:33 ` Yan Zhao
2021-09-02 9:27 ` Joerg Roedel
2021-09-02 18:41 ` Andy Lutomirski
2021-09-02 18:57 ` Sean Christopherson
2021-09-02 19:07 ` Dave Hansen
2021-09-02 20:42 ` Andy Lutomirski
2021-08-27 22:18 ` Sean Christopherson
2021-08-31 19:07 ` David Hildenbrand
2021-08-31 21:54 ` Sean Christopherson
2021-09-01 8:09 ` David Hildenbrand
2021-09-01 15:54 ` Andy Lutomirski
2021-09-01 16:16 ` David Hildenbrand
2021-09-01 17:09 ` Andy Lutomirski
2021-09-01 16:18 ` James Bottomley
2021-09-01 16:22 ` David Hildenbrand
2021-09-01 16:31 ` James Bottomley
2021-09-01 16:37 ` David Hildenbrand
2021-09-01 16:45 ` James Bottomley
2021-09-01 17:08 ` David Hildenbrand
2021-09-01 17:50 ` Sean Christopherson
2021-09-01 17:53 ` David Hildenbrand
2021-09-01 17:08 ` Andy Lutomirski
2021-09-01 17:13 ` James Bottomley
2021-09-02 10:18 ` Joerg Roedel
2021-09-01 18:24 ` Andy Lutomirski
2021-09-01 19:26 ` Dave Hansen
2021-09-07 15:00 ` Tom Lendacky
2021-09-01 4:58 ` Andy Lutomirski
2021-09-01 7:49 ` David Hildenbrand
2021-09-02 18:47 ` Kirill A. Shutemov
2021-09-02 20:33 ` Sean Christopherson
2021-09-03 19:14 ` Kirill A. Shutemov
2021-09-03 19:15 ` Andy Lutomirski
2021-09-10 17:18 ` Kirill A. Shutemov
2021-09-15 19:58 ` Chao Peng
2021-09-15 13:51 ` David Hildenbrand
2021-09-15 14:29 ` Kirill A. Shutemov
2021-09-15 14:59 ` David Hildenbrand
2021-09-15 15:35 ` David Hildenbrand
2021-09-15 20:04 ` Kirill A. Shutemov
2021-09-15 14:11 ` Kirill A. Shutemov
2021-09-16 7:36 ` Chao Peng
2021-09-16 9:24 ` Paolo Bonzini
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