Using the KVM API [LWN.net]

Using the KVM API

Posted Sep 30, 2015 6:17 UTC (Wed) by eru (subscriber, #2753)
In reply to: Using the KVM API by pbonzini
Parent article: Using the KVM API

on one hand you don't need near bare-metal performance that KVM provides, because dosemu/dosbox only need to emulate a 100 MHz machine or so, and a simple interpreter or a JIT compiler like QEMU's can handle it

Probably true for old games, but the situation I am thinking of involves using ancient cross-compilers to compiler large masses of legacy code for a weird environment that still has to be maintained. One would think (and I did think) this is an I/O-bound operation, but it turned out the speed difference between dosemu with VM86 and dosemu on x86_86 with emulation is very noticeable (order of magnitude for large inputs). On the other hand, dosemu also has advantages, because it can run "headless", can easily access native files, and starts up quickly. These are important features, because the ancient compilers are wrapped in layers that hide their MS-DOS internals, so from the Linux user's point of view they act like normal command-line tools.

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Using the KVM API

Posted Sep 30, 2015 13:46 UTC (Wed) by pbonzini (subscriber, #60935) [Link] (6 responses)

What makes you use dosemu or dosbox instead of QEMU (with or without KVM)?

Using the KVM API

Posted Sep 30, 2015 15:28 UTC (Wed) by eru (subscriber, #2753) [Link] (5 responses)

I probably should look into qemu again some day. One problem is file system access. As noted, I want the MS-DOS compilers to transparently compile sources in the Linux file system and write the objects there, and preferably without having to install any network support in the emulated MS-DOS or FreeDOS, so to leave maximum "real" memory for the compilers. Both dosemu and dosbox handle this requirement.

Using the KVM API

Posted Sep 30, 2015 17:15 UTC (Wed) by felix.s (guest, #104710) [Link] (4 responses)

There's vvfat, but QEMU people say it's deprecated, and writing (which has to be manually enabled) currently seems broken. I found a virtio-9p driver for DOS: <http://www.zytor.com/pub/git/dos/virtio9p.git/>, but it's half-done, seems abandoned, and doesn't even compile properly as of now (binutils' fault though; bug 18963). On the other hand, the missing part is mostly the "boring" stuff, which is actually translating DOS file system accesses into 9P protocol exchanges (including file name translation). Maybe you can persuade hpa to get back to it :)

Also, funnily enough, some time ago I've been working on a DOS/BIOS ABI layer based on KVM (I tried to make backends interchangeable, but I'm not sure how well I've succeeded), and I think it would be ideal for the use case you describe. I even managed to include a simplistic packet driver, so I can use the FDNPKG package manager to download programs to test. However, the code is currently such a mess that I'm too embarrassed to publish it. Maybe some day...

Using the KVM API

Posted Sep 30, 2015 17:34 UTC (Wed) by josh (subscriber, #17465) [Link] (3 responses)

I've been thinking about using the KVM API to prototype a new virtio filesystem, both for simplicity (this shouldn't need a full network filesystem) and to improve on 9p's very high access latency. (Try building a kernel under virtio-9p, and compare to host performance building the same kernel.) 9p provides reasonable throughput, but not reasonable latency. Given the ability to map guest memory, file data access could occur by mmap()ing the file on the host and then handing that memory to the guest; however, that doesn't address the performance of calls like stat(), which need to run absurdly fast and absurdly often.

Using the KVM API

Posted Sep 30, 2015 17:49 UTC (Wed) by kvaneesh (subscriber, #45646) [Link] (2 responses)

With cache=loose, we should used the cached value in the inode in getattr (v9fs_vfs_getattr_dotl). So stat should be fast.

Using the KVM API

Posted Oct 1, 2015 6:29 UTC (Thu) by kleptog (subscriber, #1183) [Link] (1 responses)

I set up a system locally to run programs under a VM in qemu-kvm with shared folders and it was insanely slow, especially git and find. Stracing on the host showed that for each stat() in the guest, the host was doing a stat() on each directory component. So a stat() of a complete directory tree went O(N^2) on the directory depth.

Now, I get that it's probably a configuration issue since it clearly wasn't caching anything, but I found it really hard to find documentation about qemu that explained this behaviour. On top of that I'm managing them via libvirt, so even if I find a command-line option to deal with something, if libvirt doesn't support it I'm still SOL.

Overall, it hasn't been a great experience, next time I'll probably do what other people do, use VirtualBox or VMWare.

But back to the article, it's a pretty nice interface actually. Hopefully I'll find some reason to use it sometime :)

Using the KVM API

Posted Oct 8, 2015 16:48 UTC (Thu) by LightDot (guest, #73140) [Link]

You can pass raw qemu command line in the libvirt XML. This is an equivalent of '-vnc :30,tls -k fr -no-fd-bootchk':

The VNC option doesn't need to be presented as a command line, I just left it as an example.