On 7/27/26 08:13, Peter Maydell wrote:
When we implememented SEV to do something on A-profile rather than
being a nop, we got the condition slightly wrong, and made it only
effective from v7. In fact the instruction's Arm encoding has
non-NOP behaviour from ARMv6K.
The effect is that a kernel boot may hang on a v6K CPU like the
ARM11MPCore.
(This wouldn't have been so noticeable if we feature checked the WFE
instruction, and had made the same mistake for the condition on both
instructions. But we never have done the feature checks that we
ought on WFE, so the mistake on SEV meant that we showed the 11mpcore
guest a WFE that did something and a SEV that was a NOP.)
The v7A Arm ARM is not entirely clear about whether v6K has the Thumb
SEV encoding or not: it says "ARMv7 (executes as NOP in ARMv6T2)",
leaving v6K not stated. The 11MPCore TRM says it has at least WFI in
both Arm and Thumb, and the v7A Arm ARM uses the same condition text
for WFI, so I make the assumption that WFI, WFE, and SEV all get
their functionality for both Thumb and Arm in v6K. It's possible
that this differed between v6K CPUs -- the 1176 TRM says it has the
v6K STREXD/STREXH/STREXB etc, but the WFI is the old-style cp15 one.
Keeping the condition check the same for both Thumb and Arm encodings
is the conservative choice: if guests try to execute the Thumb SEV
insn it will be because they want SEV, not because they want a NOP.
Resolves:https://gitlab.com/qemu-project/qemu/-/work_items/4044
Fixes: 60e7ee5bb7cd ("target/arm: implements SEV/SEVL for all modes")
Signed-off-by: Peter Maydell<[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Picking this one up because I think we should fix it for 11.1
and Alex is on holiday. The "WFI and WFE should have a feature
check" part we can leave for 11.2.
Really? I'm tempted to call the mismatch between wait and signal to be the
"real" bug.
r~
