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Currently, BPF_CALL is always jited to indirect call. When target is
within the range of direct call, BPF_CALL can be jited to direct call.
For example, the following BPF_CALL
call __htab_map_lookup_elem
is always jited to indirect call:
mov x10, #0xffffffffffff18f4
movk x10, #0x821, lsl #16
movk x10, #0x8000, lsl #32
blr x10
When the address of target __htab_map_lookup_elem is within the range of
direct call, the BPF_CALL can be jited to:
bl 0xfffffffffd33bc98
This patch does such jit optimization by emitting arm64 direct calls for
BPF_CALL when possible, indirect calls otherwise.
Without this patch, the jit works as follows.
1. First pass
A. Determine jited position and size for each bpf instruction.
B. Computed the jited image size.
2. Allocate jited image with size computed in step 1.
3. Second pass
A. Adjust jump offset for jump instructions
B. Write the final image.
This works because, for a given bpf prog, regardless of where the jited
image is allocated, the jited result for each instruction is fixed. The
second pass differs from the first only in adjusting the jump offsets,
like changing "jmp imm1" to "jmp imm2", while the position and size of
the "jmp" instruction remain unchanged.
Now considering whether to jit BPF_CALL to arm64 direct or indirect call
instruction. The choice depends solely on the jump offset: direct call
if the jump offset is within 128MB, indirect call otherwise.
For a given BPF_CALL, the target address is known, so the jump offset is
decided by the jited address of the BPF_CALL instruction. In other words,
for a given bpf prog, the jited result for each BPF_CALL is determined
by its jited address.
The jited address for a BPF_CALL is the jited image address plus the
total jited size of all preceding instructions. For a given bpf prog,
there are clearly no BPF_CALL instructions before the first BPF_CALL
instruction. Since the jited result for all other instructions other
than BPF_CALL are fixed, the total jited size preceding the first
BPF_CALL is also fixed. Therefore, once the jited image is allocated,
the jited address for the first BPF_CALL is fixed.
Now that the jited result for the first BPF_CALL is fixed, the jited
results for all instructions preceding the second BPF_CALL are fixed.
So the jited address and result for the second BPF_CALL are also fixed.
Similarly, we can conclude that the jited addresses and results for all
subsequent BPF_CALL instructions are fixed.
This means that, for a given bpf prog, once the jited image is allocated,
the jited address and result for all instructions, including all BPF_CALL
instructions, are fixed.
Based on the observation, with this patch, the jit works as follows.
1. First pass
Estimate the maximum jited image size. In this pass, all BPF_CALLs
are jited to arm64 indirect calls since the jump offsets are unknown
because the jited image is not allocated.
2. Allocate jited image with size estimated in step 1.
3. Second pass
A. Determine the jited result for each BPF_CALL.
B. Determine jited address and size for each bpf instruction.
4. Third pass
A. Adjust jump offset for jump instructions.
B. Write the final image.
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Reviewed-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20240903094407.601107-1-xukuohai@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.