Re: [Qemu-devel] [PULL 08/13] target/arm: Pull Thumb insn word loads up to top level

[Emilio G. Cota]

On Thu, Oct 12, 2017 at 17:03:31 +0100, Peter Maydell wrote:
> Refactor the Thumb decode to do the loads of the instruction words at
> the top level rather than only loading the second half of a 32-bit
> Thumb insn in the middle of the decode.
> 
> This is simple apart from the awkward case of Thumb1, where the
> BL/BLX prefix and suffix instructions live in what in Thumb2 is the
> 32-bit insn space.  To handle these we decode enough to identify
> whether we're looking at a prefix/suffix that we handle as a 16 bit
> insn, or a prefix that we're going to merge with the following suffix
> to consider as a 32 bit insn.  The translation of the 16 bit cases
> then moves from disas_thumb2_insn() to disas_thumb_insn().
> 
> The refactoring has the benefit that we don't need to pass the
> CPUARMState* down into the decoder code any more, but the major
> reason for doing this is that some Thumb instructions must be always
> unconditional regardless of the IT state bits, so we need to know the
> whole insn before we emit the "skip this insn if the IT bits and cond
> state tell us to" code.  (The always unconditional insns are BKPT,
> HLT and SG; the last of these is 32 bits.)
> 
> Signed-off-by: Peter Maydell <address@hidden>
> Reviewed-by: Richard Henderson <address@hidden>
> Message-id: address@hidden

This commit breaks the debian-arm boot test (see [1]), boot dies at:

> random: systemd urandom read with 4 bits of entropy available
> systemd[1]: Caught <SEGV>, core dump failed.
> systemd[1]: Freezing execution.

Sorry I noticed this so late (-rc4), for a while I thought my development
was causing this and didn't pay much attention to it. However, just realised
the problem is present on master. Bisect log below.

Thanks,

                Emilio

[1] https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg04085.html

$ git bisect log
git bisect start
# bad: [2babfe0c9241c239272a03fec785165a50e8288c] Update version for 
v2.11.0-rc4 release
git bisect bad 2babfe0c9241c239272a03fec785165a50e8288c
# good: [1ab5eb4efb91a3d4569b0df6e824cc08ab4bd8ec] Update version for v2.10.0 
release
git bisect good 1ab5eb4efb91a3d4569b0df6e824cc08ab4bd8ec
# good: [62a2554ec2630896d1299e1a282a64c7f3b00da0] 390x/css: introduce maximum 
data address checking
git bisect good 62a2554ec2630896d1299e1a282a64c7f3b00da0
# bad: [f51f315a676ec913a55ac27be4ef857f9f7ddc5c] translate-all: use 
qemu_protect_rwx/none helpers
git bisect bad f51f315a676ec913a55ac27be4ef857f9f7ddc5c
# bad: [9f99c85c4a364f8de8134eb53b0cc1b84ded4b3f] Merge remote-tracking branch 
'remotes/gkurz/tags/for-upstream' into staging
git bisect bad 9f99c85c4a364f8de8134eb53b0cc1b84ded4b3f
# good: [3637cf58f9441ad277fd70299a29d0e39b32c96c] util: move 
qemu_real_host_page_size/mask to osdep.h
git bisect good 3637cf58f9441ad277fd70299a29d0e39b32c96c
# bad: [b81b948ecc8659d78066f374c787ed12379d21dd] virtio/pci/migration: Convert 
to VMState
git bisect bad b81b948ecc8659d78066f374c787ed12379d21dd
# good: [43851b5bd48d952561610d0d6d6c314c97eff543] iotests: Set up Python 
logging
git bisect good 43851b5bd48d952561610d0d6d6c314c97eff543
# bad: [76eff04d166b8fe747adbe82de8b7e060e668ff9] target/arm: Implement SG 
instruction corner cases
git bisect bad 76eff04d166b8fe747adbe82de8b7e060e668ff9
# good: [b9f587d62cebed427206539750ebf59bde4df422] target/arm: Add M profile 
secure MMU index values to get_a32_user_mem_index()
git bisect good b9f587d62cebed427206539750ebf59bde4df422
# good: [6b8acf256df09c8a8dd7dcaa79b06eaff4ad63f7] target-arm: Don't check for 
"Thumb2 or M profile" for not-Thumb1
git bisect good 6b8acf256df09c8a8dd7dcaa79b06eaff4ad63f7
# bad: [5b8d7289e9e92a0d7bcecb93cd189e245fef10cd] target-arm: Simplify 
insn_crosses_page()
git bisect bad 5b8d7289e9e92a0d7bcecb93cd189e245fef10cd
# bad: [296e5a0a6c393553079a641c50521ae33ff89324] target/arm: Pull Thumb insn 
word loads up to top level
git bisect bad 296e5a0a6c393553079a641c50521ae33ff89324
# first bad commit: [296e5a0a6c393553079a641c50521ae33ff89324] target/arm: Pull 
Thumb insn word loads up to top level