add a C compatible interface #43
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rageagainsthepc
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I added some comments over your work so far.
I've tested it on my machine and it compiled and worked for me, except for the mem-dump.
We should also provide a Makefile in c_examples, automatically compiling and linking with libmicrovmi in $root/target/debug.
A c_examples/README will be useful as well, explaining how to generate the libmicrovmi.h here, and how to set your LD_LIBRARY_PATH.
Congrats for all your work @rageagainsthepc ! 🎉 👍
rageagainsthepc
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c_examples/README.md
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| Command to execute an example as an unprivileged user: | ||
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| ~~~ | ||
| sudo LD_LIBRARY_PATH="$LD_LIBRARY_PATH:../target/debug" <example> <vm_name> |
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sudo 🤔
If you are working with Xen, you need to be root, that's correct.
But in the general case, I would prefer to remove this indication, since it's not needed for KVM and VirtualBox.
| @@ -64,4 +67,6 @@ pub trait Introspectable { | |||
| fn resume(&mut self) -> Result<(), Box<dyn Error>> { | |||
| unimplemented!(); | |||
| } | |||
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| fn get_driver_type(&self) -> DriverType; | |||
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@rageagainsthepc can you explain why you had to introduce a new API here, it's hard to follow for me ?
I would like to avoid changing the core of the Rust library just to provide a C compatibility layer.
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Well, if we initialize the driver with None, we don't know the type of the driver in advance. But we need to know since we are casting back and forth between void* and the actual type of the driver. As far as I know it's not possible to cast directly from void* to a trait type. There are probably different ways to approach this, but this one seemed pretty straight forward to me. I'm open to alternatives though ;)
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I think the issue was that pointers to traits are fat pointers. Maybe there is way to pass fat pointers to C code and cast them back to traits afterwards. I'll look into that.
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There exists a std::raw::TraitObject but it's marked as unstable because its representation may change in the future rust-lang/rust#27751 . Also cbindgen reports TraitObject types as unsupported. In theory we could split the fat pointer into two separate pointers, store them in a struct and transmute the memory back to an Introspectable, but imho that sounds super ugly and unstable.
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On the other hand, we could just loose the support for passing None instead of the DriverType, at least for the C-API ;)
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I've found the crate downcast-rs. With this crate you are able to check a trait object for its concrete type.
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Okay, thanks for the update.
I'm okay to merge this as long as you add a comment that this API has been introduced for the solve purpose of the C interoperability, and that it's a workaround, to be deprecated in the future when new better solutions will emerge to deal with this, either from Rust unstable, or simply by switching to dynamic library loading.
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| @@ -64,4 +67,6 @@ pub trait Introspectable { | |||
| fn resume(&mut self) -> Result<(), Box<dyn Error>> { | |||
| unimplemented!(); | |||
| } | |||
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| fn get_driver_type(&self) -> DriverType; | |||
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Okay, thanks for the update.
I'm okay to merge this as long as you add a comment that this API has been introduced for the solve purpose of the C interoperability, and that it's a workaround, to be deprecated in the future when new better solutions will emerge to deal with this, either from Rust unstable, or simply by switching to dynamic library loading.
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