Hi,
Any comment?
Cheers,
Wei Chen
> -----Original Message-----
> From: Wei Chen
> Sent: 2021年6月15日 14:12
> To: 'kvm@xxxxxxxxxxxxxxx' <kvm@xxxxxxxxxxxxxxx>; xen-devel@xxxxxxxxxxxxx
> Cc: 'will@xxxxxxxxxx' <will@xxxxxxxxxx>; 'jean-philippe@xxxxxxxxxx' <jean-
> philippe@xxxxxxxxxx>; 'Julien Grall' <julien@xxxxxxx>; Andre Przywara
> <Andre.Przywara@xxxxxxx>; 'Marc Zyngier' <maz@xxxxxxxxxx>;
> 'julien.thierry.kdev@xxxxxxxxx' <julien.thierry.kdev@xxxxxxxxx>; Stefano
> Stabellini <sstabellini@xxxxxxxxxx>; 'Oleksandr Tyshchenko'
> <Oleksandr_Tyshchenko@xxxxxxxx>
> Subject: [Kvmtool] Some thoughts on using kvmtool Virtio for Xen
>
> Hi,
>
> I have some thoughts of using kvmtool Virtio implementation
> for Xen. I copied my markdown file to this email. If you have
> time, could you please help me review it?
>
> Any feedback is welcome!
>
> # Some thoughts on using kvmtool Virtio for Xen
> ## Background
>
> Xen community is working on adding VIRTIO capability to Xen. And we're
> working
> on VIRTIO backend of Xen. But except QEMU can support virtio-net for x86-
> xen,
> there is not any VIRTIO backend can support Xen. Because of the
> community's
> strong voice of Out-of-QEMU, we want to find a light weight VIRTIO backend
> to
> support Xen.
>
> We have an idea of utilizing the virtio implementaton of kvmtool for Xen.
> And
> We know there was some agreement that kvmtool won't try to be a full QEMU
> alternative. So we have written two proposals in following content for
> communities to discuss in public:
>
> ## Proposals
> ### 1. Introduce a new "dm-only" command
> 1. Introduce a new "dm-only" command to provide a pure device model mode.
> In
> this mode, kvmtool only handles IO request. VM creation and
> initialization
> will be bypassed.
>
> * We will rework the interface between the virtio code and the rest of
> kvmtool, to use just the minimal set of information. At the end, there
> would be MMIO accesses and shared memory that control the device model,
> so that could be abstracted to do away with any KVM specifics at all.
> If
> this is workable, we will send the first set of patches to introduce
> this
> interface, and adapt the existing kvmtool to it. Then later we will
> can
> add Xen support on top of it.
>
> About Xen support, we will detect the presence of Xen libraries, also
> allow people to ignore them, as kvmtoll do with optional features like
> libz or libaio.
>
> Idealy, we want to move all code replying on Xen libraries to a set of
> new files. In this case, thes files can only be compiled when Xen
> libraries are detected. But if we can't decouple this code completely,
> we may introduce a bit of #ifdefs to protect this code.
>
> If kvm or other VMM do not need "dm-only" mode. Or "dm-only" can not
> work without Xen libraries. We will make "dm-only" command depends on
> the presence of Xen libraries.
>
> So a normal compile (without the Xen libraries installed) would create
> a binary as close as possible to the current code, and only the people
> who having Xen libraries installed would ever generate a "dm-only"
> capable kvmtool.
>
> ### 2. Abstract kvmtool virtio implementation as a library
> 1. Add a kvmtool Makefile target to generate a virtio library. In this
> scenario, not just Xen, but any project else want to provide a
> userspace virtio backend service can link to this virtio libraris.
> These users would benefit from the VIRTIO implementation of kvmtool
> and will participate in improvements, upgrades, and maintenance of
> the VIRTIO libraries.
>
> * In this case, Xen part code will not upstream to kvmtool repo,
> it would then be natural parts of the xen repo, in xen/tools or
> maintained in other repo.
>
> We will have a completely separate VIRTIO backend for Xen, just
> linking to kvmtool's VIRTIO library.
>
> * The main changes of kvmtool would be:
> 1. Still need to rework the interface between the virtio code
> and the rest of kvmtool, to abstract the whole virtio
> implementation into a library
> 2. Modify current build system to add a new virtio library target.
>
> ## Reworking the interface is the common work for above proposals
> **In kvmtool, one virtual device can be separated into three layers:**
>
> - A device type layer to provide an abstract
> - Provide interface to collect and store device configuration.
> Using block device as an example, kvmtool is using disk_image to
> - collect and store disk parameters like:
> - backend image format: raw, qcow or block device
> - backend block device or file image path
> - Readonly, direct and etc
> - Provide operations to interact with real backend devices or services:
> - provide backend device operations:
> - block device operations
> - raw image operations
> - qcow image operations
> - Hypervisor interfaces
> - Guest memory mapping and unmapping interfaces
> - Virtual device register interface
> - MMIO/PIO space register
> - IRQ register
> - Virtual IRQ inject interface
> - Hypervisor eventfd interface
> - An implementation layer to handle guest IO request.
> - Kvmtool provides virtual devices for guest. Some virtual devices two
> kinds of implementations:
> - VIRTIO implementation
> - Real hardware emulation
>
> For example, kvmtool console has virtio console and 8250 serial two kinds
> of implementations. These implementation depends on device type parameters
> to create devices, and depends on device type ops to forward data from/to
> real device. And the implementation will invoke hypervisor interfaces to
> map/unmap resources and notify guest.
>
> In the current kvmtool code, the boundaries between these three layers are
> relatively clear, but there are a few pieces of code that are somewhat
> interleaved, for example:
> - In virtio_blk__init(...) function, the code will use disk_image directly.
> This data is kvmtool specified. If we want to make VIRTIO implementation
> become hypervisor agnostic. Such kind of code should be moved to other
> place. Or we just keep code from virtio_blk__init_one(...) in virtio
> block
> implementation, but keep virtio_blk__init(...) in kvmtool specified part
> code.
>
> However, in the current VIRTIO device creation and data handling process,
> the device type and hypervisor API used are both exclusive to kvmtool and
> KVM. If we want to use current VIRTIO implementation for other device
> models and hypervisors, it is unlikely to work properly.
>
> So, the major work of reworking interface is decoupling VIRTIO
> implementation
> from kvmtool and KVM.
>
> **Introduce some intermediate data structures to do decouple:**
> 1. Introduce intermedidate type data structures like `virtio_disk_type`,
> `virtio_net_type`, `virtio_console_type` and etc. These data structures
> will be the standard device type interfaces between virtio device
> implementation and hypervisor. Using virtio_disk_type as an example:
> ~~~~
> struct virtio_disk_type {
> /*
> * Essential configuration for virtio block device can be got from
> * kvmtool disk_image. Other hypervisor device model also can use
> * this data structure to pass necessary parameters for creating
> * a virtio block device.
> */
> struct virtio_blk_cfg vblk_cfg;
> /*
> * Virtio block device MMIO address and IRQ line. These two
> members
> * are optional. If hypervisor provides allocate_mmio_space and
> * allocate_irq_line capability and device model doesn't set these
> * two fields, virtio block implementation will use hypervisor
> APIs
> * to allocate MMIO address and IRQ line. If these two fields are
> * configured, virtio block implementation will use them.
> */
> paddr_t addr;
> uint32_t irq;
> /*
> * In kvmtool, this ops will connect to disk_image APIs. Other
> * hypervisor device model should provide similar APIs for this
> * ops to interact with real backend device.
> */
> struct disk_type_ops {
> .read
> .write
> .flush
> .wait
> ...
> } ops;
> };
> ~~~~
>
> 2. Introduce a intermediate hypervisor data structure. This data structure
> provides a set of standard hypervisor API interfaces. In virtio
> implementation, the KVM specified APIs, like kvm_register_mmio, will
> not
> be invoked directly. The virtio implementation will use these
> interfaces
> to access hypervisor specified APIs. for example `struct vmm_impl`:
> ~~~~
> struct vmm_impl {
> /*
> * Pointer that link to real hypervisor handle like `struct kvm
> *kvm`.
> * This pointer will be passed to the vmm ops;
> */
> void *vmm;
> allocate_irq_line_fn_t(void* vmm, ...);
> allocate_mmio_space_fn_t(void* vmm, ...);
> register_mmio_fn_t(void* vmm, ...);
> map_guest_page_fn_t(void* vmm, ...);
> unmap_guest_page_fn_t(void* vmm, ...);
> virtual_irq_inject_fn_t(void* vmm, ...);
> };
> ~~~~
>
> 3. After decoupled with kvmtool, any hypervisor can use standard
> `vmm_impl`
> and `virtio_xxxx_type` interfaces to invoke standard virtio
> implementation
> interfaces to create virtio devices.
> ~~~~
> /* Prepare VMM interface */
> struct vmm_impl *vmm = ...;
> vmm->register_mmio_fn_t = kvm__register_mmio;
> /* kvm__map_guset_page is a wrapper guest_flat_to_host */
> vmm->map_guest_page_fn_t = kvm__map_guset_page;
> ...
>
> /* Prepare virtio_disk_type */
> struct virtio_disk_type *vdisk_type = ...;
> vdisk_type->vblk_cfg.capacity = disk_image->size / SECTOR_SIZE;
> ...
> vdisk_type->ops->read = disk_image__read;
> vdisk_type->ops->write = disk_image__write;
> ...
>
> /* Invoke VIRTIO implementation API to create a virtio block device */
> virtio_blk__init_one(vmm, vdisk_type);
> ~~~~
>
> VIRTIO block device simple flow before reworking interface:
> https://drive.google.com/file/d/1k0Grd4RSuCmhKUPktHj9FRamEYrPCFkX/view?usp
> =sharing
> 
>
> VIRTIO block device simple flow after reworking interface:
> https://drive.google.com/file/d/1rMXRvulwlRO39juWf08Wgk3G1NZtG2nL/view?usp
> =sharing
> 
>
>
> Thanks,
> Wei Chen
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