Friday, 2021-05-21

kata-irc-bot1<cole.mickens> yeah the rev I'm building seems to include that, iiuc01:19
kata-irc-bot1<cole.mickens> how would I run virtiofsd manually in order to be able to run the full qemu command that kata-runtime invokes?01:21
kata-irc-bot1<lingniao> https://virtio-fs.gitlab.io/howto-qemu.html01:23
kata-irc-bot1<liubin0329> You can also enable debug logs and check the output by grepping virtiofsd for command path and arguments https://github.com/kata-containers/kata-containers/blob/main/src/runtime/virtcontainers/virtiofsd.go#L114-L11505:59
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kata-irc-bot1<lingniao> I have several questions, I hope to think about it clearly 1. Does dax access the read-only content of images or the entire virtiofs data? 2. Cache mode seems to be dax’s refresh strategy. Then the guest wrote the data file. Is there any consistency problem? 3. Dax shares the memory of the host machine in the same area, one for each VM. If it is one, if a large number of VMs are used at the same time, will there be a problem of07:09
kata-irc-bot1competition? 4. Why can’t the virtiofs source be set to the hard disk directory, only the /run/ memory directory? 5. Every virtiofs process uses the same socket descriptor with the same FD=3. Will this be a problem?07:09
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kata-irc-bot1<harshanavkis> Hello everyone! I'm Harshavardhan Unnibhavi (you can call me Harsha). I am one of QEMU's gsoc students for this year. I will be working with Fabiano Fidêncio and Stefano Garzarella on developing a vhost-user-vsock application in rust and integrating it with Kata Container!09:57
kata-irc-bot1<fidencio> Welcome, @harshanavkis!12:02
kata-irc-bot1<fidencio> cc @christophe12:03
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kata-irc-bot1<christophe> Let me start with 5, the easiest. File descriptors are per process, stdio occupying 0, 1 and 2, which is why we always share 3, the first free fd. So not a problem.15:20
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kata-irc-bot1<christophe> 4. Not sure what you call a "hard disk directory". virtiofs uses shared memory, so that has to be memory, not disk storage. Maybe I misunderstood the question15:21
kata-irc-bot1<christophe> 3. I'm not entirely sure what you mean either. I think that you are referring to the memory mapping. The virtual addresses may be similar from one VM to the next, but the underlying buffers are unlikely to be..15:22
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kata-irc-bot1<christophe> 2. There are multiple caching strategies, but they mostly relate to meta-data IIRC. So there may be consistency problems for things like file locking, which is why there are options. The tradeoff is between consistency and performance. I don't think DAX changes much there, but I will check.  See https://lwn.net/Articles/828371/ for some of the limitations with DAX with respect to sharing. See15:29
kata-irc-bot1https://qemu-stsquad.readthedocs.io/en/docs-next/tools/virtiofsd.html for the options to `-o cache=` you can pass to virtiofs.15:29
kata-irc-bot1<christophe> 1. DAX is only a way to share the host file buffer cache. Say you want to access file `/foo/bar`  in your guest that corresponds to `/some/guest/foo/bar` on the host. When you read the file, it gets cached by the host kernel. Without DAX, you'd then transfer the data to the guest, which would make another copy in the guest file buffer cache. So you'd have two copies of the same data, and more copy operations to get the result,15:32
kata-irc-bot1With DAX, you instead map the host page representing the file data into the guest memory space. So no additional copy is needed. This si the reason for some of the limitations outlined in the LWN article I linked above (e.g. multiple guests can't use the same file that way).15:32
kata-irc-bot1<christophe> HTH @lingniao ^15:32
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