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Re: [libvirt] [PATCH v2 03/17] vircgroup: introduce virCgroupV2DevicesAttachProg



On Tue, Jan 15, 2019 at 09:58:47AM +0100, Pavel Hrdina wrote:
> On Mon, Jan 14, 2019 at 04:43:34PM +0000, Daniel P. Berrangé wrote:
> > On Mon, Jan 14, 2019 at 04:47:37PM +0100, Pavel Hrdina wrote:
> > > This function loads the BPF prog with prepared map into kernel and
> > > attaches it into guest cgroup.  It can be also used to replace existing
> > > program in the cgroup if we need to resize BPF map to store more rules
> > > for devices. The old program will be closed and removed from kernel.
> > > 
> > > There are two possible ways how to create BPF program:
> > > 
> > >     - One way is to write simple C-like code which can by compiled into
> > >       BPF object file which can be loaded into kernel using elfutils.
> > > 
> > >     - The second way is to define macros which looks like assembler
> > >       instructions and can be used directly to create BPF program that
> > >       can be directly loaded into kernel.
> > > 
> > > Since the program is not too complex we can use the second option.
> > 
> > I'm really not liking this to be honest. Even for this "simple" program,
> > I struggle to understand what the code below is doing. It is basically
> > assembly language, which is inevitably hard to follow even for simple
> > things.
> > 
> > I'd like to see us use the BPF C compiler to build the loaded
> > program unless there's a compelling reason why it won't work
> 
> I personally don't like it as well, but the issue is that for the
> compilation we would have to use clang/llvm, AFAIK there is no support
> to compile BPF in GCC.  Another issue is the loading part, you need to
> read the object file, extract correct sections which is not trivial and
> in order to have different size of maps we would have to modify the
> loaded data from the compiled object file.

I don't see a problem with using clang for BPF.  Reading the DPDK lists,
I see you can do something like this:

  $ clang -I iproute2/include/ -O2 -emit-llvm -c tap_bpf_program.c -o - \
        | llc -march=bpf -filetype=obj -o tap_bpf_program.o
  $ objdump -j l3_l4 -s tap_bpf_program.o > tap_bpf_program.bin

where the tap_bpf_program.bin should be the raw instructions to be
loaded.

Supposedly they also have a script that turns the .bin file into a
C byte array which they then compile into their binary, so they
can just load the uint8[] directly, instead of from a separate file

The matter of dynamically sized device maps is more tricky. Assuming
the device map size is basically a list of (major,minor) numbers,
that's 2 bytes per entry. We could easily do a fixed size 50 elements
giving 100 bytes per guest if that makes it easier. I guess the hard
thing is actually getting the dynamic data into the code.

> I can try to prepare alternative patches for that but it might me even
> more complex to understand than short assembly program.

Maybe there's a compromise/hybrid that would work simpler ? eg is
it possible to structure things so all the checking code is
statically compiled, and then we dynamically generate the instructions
to define the list of devices and append that to the main static
code. ?

Regards,
Daniel
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