Re: [Qemu-devel] [PATCH v3 2/2] docs: update ivshmem device spec
On 09/01/2014 03:52 AM, David Marchand wrote: What about upgrading QEMU and ivshmem-server while you have existing guests? You cannot restart ivshmem-server, and the new QEMU would have to talk to the old ivshmem-server. Version negotiation also helps avoid confusion if someone combines ivshmem-server and QEMU from different origins (e.g. built from source and distro packaged). Don't underestimate the likelihood of this happening. Any long-running process (which an ivshmem-server will be) continues running at the old version, even when a package upgrade installs a new qemu binary; the new binary should still be able to manage connections to the already-running server. Even neater would be a solution where an existing ivshmem-server could re-exec an updated ivshmem-server binary that resulted from a distro upgrade, hand over all state required for the new server to take over from the point managed by the old server, so that you aren't stuck running the old binary forever. But that's a lot trickier to write, so it is not necessary for a first implementation; and if you do that, then you have the reverse situation to worry about (the new server must still accept communication from existing old qemu binaries). Note that the goal here is to support upgrades; it is probably okay if downgrading from a new binary back to an old doesn't work correctly (because the new software was using a feature not present in the old). It's a safeguard to prevent hard-to-diagnose failures when the system is misconfigured. Hum, so you want the code to be defensive against mis-use, why not. I wanted to keep modifications on ivshmem as little as possible in a first phase (all the more so as there are potential ivshmem users out there that I think will be impacted by a protocol change). Existing ivshmem users MUST be aware that they are using something that is not yet polished, and be prepared to make the upgrade to the polished version. It's best to minimize the hassle by making them upgrade exactly once to a fully-robust version, rather than to have them upgrade to a slightly-more robust version only to find out we didn't plan ahead well enough to make further extensions in a back-compatible manner. Sending the version as the first vm_id with an associated fd to -1 before sending the real client id should work with existing QEMU client code (hw/misc/ivshmem.c). Do you have a better idea ? Is there a best practice in QEMU for version negotiation that could work with ivshmem protocol ? QMP starts off with a mandatory qmp_capabilities handshake, although we haven't yet had to define any capabilities where cross-versioned communication differs as a result. Migration is somewhat of an example, except that it is one-directional (we don't have a feedback path), so it is somewhat best effort. The qcow2 v3 file format is an example of declaring features, rather than version numbers, and making decisions about whether a feature is compatible (older clients can safely ignore the bit, without corrupting the image but possibly having worse performance) vs. incompatible (older clients must reject the image, because not handling the feature correctly would corrupt the image). The best handshakes are bi-directional - both sides advertise their version (or better, their features), and a well-defined algorithm for settling on the common subset of advertised features then ensures that the two sides know how to talk to each other, or give a reason for either side to disconnect early because of a missing feature. I have a v4 ready with this (and all the pending comments), I will send it later unless a better idea is exposed. Thanks. -- Eric Blake eblake redhat com+1-919-301-3266 Libvirt virtualization library http://libvirt.org signature.asc Description: OpenPGP digital signature
Re: [Qemu-devel] [PATCH v3 2/2] docs: update ivshmem device spec
On 08/28/2014 11:49 AM, Stefan Hajnoczi wrote: On Tue, Aug 26, 2014 at 01:04:30PM +0200, Paolo Bonzini wrote: Il 26/08/2014 08:47, David Marchand ha scritto: Using a version message supposes we want to keep ivshmem-server and QEMU separated (for example, in two distribution packages) while we can avoid this, so why would we do so ? If we want the ivshmem-server to come with QEMU, then both are supposed to be aligned on your system. What about upgrading QEMU and ivshmem-server while you have existing guests? You cannot restart ivshmem-server, and the new QEMU would have to talk to the old ivshmem-server. Version negotiation also helps avoid confusion if someone combines ivshmem-server and QEMU from different origins (e.g. built from source and distro packaged). It's a safeguard to prevent hard-to-diagnose failures when the system is misconfigured. Hum, so you want the code to be defensive against mis-use, why not. I wanted to keep modifications on ivshmem as little as possible in a first phase (all the more so as there are potential ivshmem users out there that I think will be impacted by a protocol change). Sending the version as the first vm_id with an associated fd to -1 before sending the real client id should work with existing QEMU client code (hw/misc/ivshmem.c). Do you have a better idea ? Is there a best practice in QEMU for version negotiation that could work with ivshmem protocol ? I have a v4 ready with this (and all the pending comments), I will send it later unless a better idea is exposed. Thanks. -- David Marchand -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Qemu-devel] [PATCH v3 2/2] docs: update ivshmem device spec
On Tue, Aug 26, 2014 at 01:04:30PM +0200, Paolo Bonzini wrote: Il 26/08/2014 08:47, David Marchand ha scritto: Using a version message supposes we want to keep ivshmem-server and QEMU separated (for example, in two distribution packages) while we can avoid this, so why would we do so ? If we want the ivshmem-server to come with QEMU, then both are supposed to be aligned on your system. What about upgrading QEMU and ivshmem-server while you have existing guests? You cannot restart ivshmem-server, and the new QEMU would have to talk to the old ivshmem-server. Version negotiation also helps avoid confusion if someone combines ivshmem-server and QEMU from different origins (e.g. built from source and distro packaged). It's a safeguard to prevent hard-to-diagnose failures when the system is misconfigured. Stefan pgpFp_zSoZiZ7.pgp Description: PGP signature
Re: [Qemu-devel] [PATCH v3 2/2] docs: update ivshmem device spec
Hello Stefan, On 08/08/2014 05:02 PM, Stefan Hajnoczi wrote: On Fri, Aug 08, 2014 at 10:55:18AM +0200, David Marchand wrote: +For each client (QEMU processes) that connects to the server: +- the server assigns an ID for this client and sends this ID to him as the first + message, +- the server sends a fd to the shared memory object to this client, +- the server creates a new set of host eventfds associated to the new client and + sends this set to all already connected clients, +- finally, the server sends all the eventfds sets for all clients to the new + client. The protocol is not extensible and no version number is exchanged. For the most part this should be okay because clients must run on the same machine as the server. It is assumed clients and server are compatible with each other. I wonder if we'll get into trouble later if the protocol needs to be extended or some operation needs to happen, like upgrading QEMU or the ivshmem-server. At the very least someone building from source but using system QEMU or ivshmem-server could get confusing failures if the protocol doesn't match. How about sending a version message as the first thing during a connection? I am not too sure about this. This would break current base version. Using a version message supposes we want to keep ivshmem-server and QEMU separated (for example, in two distribution packages) while we can avoid this, so why would we do so ? If we want the ivshmem-server to come with QEMU, then both are supposed to be aligned on your system. If you want to test local modifications, then it means you know what you are doing and you will call the right ivshmem-server binary with the right QEMU binary. Thanks. -- David Marchand -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Qemu-devel] [PATCH v3 2/2] docs: update ivshmem device spec
Il 26/08/2014 08:47, David Marchand ha scritto: Using a version message supposes we want to keep ivshmem-server and QEMU separated (for example, in two distribution packages) while we can avoid this, so why would we do so ? If we want the ivshmem-server to come with QEMU, then both are supposed to be aligned on your system. What about upgrading QEMU and ivshmem-server while you have existing guests? You cannot restart ivshmem-server, and the new QEMU would have to talk to the old ivshmem-server. Paolo If you want to test local modifications, then it means you know what you are doing and you will call the right ivshmem-server binary with the right QEMU binary. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Qemu-devel] [PATCH v3 2/2] docs: update ivshmem device spec
On Fri, Aug 08, 2014 at 10:55:18AM +0200, David Marchand wrote: +For each client (QEMU processes) that connects to the server: +- the server assigns an ID for this client and sends this ID to him as the first + message, +- the server sends a fd to the shared memory object to this client, +- the server creates a new set of host eventfds associated to the new client and + sends this set to all already connected clients, +- finally, the server sends all the eventfds sets for all clients to the new + client. The protocol is not extensible and no version number is exchanged. For the most part this should be okay because clients must run on the same machine as the server. It is assumed clients and server are compatible with each other. I wonder if we'll get into trouble later if the protocol needs to be extended or some operation needs to happen, like upgrading QEMU or the ivshmem-server. At the very least someone building from source but using system QEMU or ivshmem-server could get confusing failures if the protocol doesn't match. How about sending a version message as the first thing during a connection? Stefan pgpQCEccZfgg9.pgp Description: PGP signature
