https://bugs.kde.org/show_bug.cgi?id=386203
Bug ID: 386203
Summary: Disable RTM cpuid bit by default
Product: valgrind
Version: unspecified
Platform: Other
OS: Linux
Status: UNCONFIRMED
Severity: normal
Priority: NOR
Component: general
Assignee: jsew...@acm.org
Reporter: m...@klomp.org
Target Milestone: ---
This is kind of the reverse/dual of bug #324882 (CPUID should depend on VEX
capabilities, not on underlying CPU) and bug #211499 (Option to return values
from processor's native cpuid instruction). The meta request would be to make
cpuid selection/emulation more flexible.
Currently if we select the amd64g_dirtyhelper_CPUID_avx_and_cx16 (but not for
amd64g_dirtyhelper_CPUID_avx2?) we claim to support RTM (restructed transactual
memory) by setting bit 11 for AEX=7 (extended feature flags).
So it might happen that we say we support HTM even if the underlying processor
doesn't.
This is technically correct since VEX does always support xtest/xbegin/xend.
But the implementation is kind of bogus. It always immediately aborts the
transaction. So while it is good for testing your code works fine under that
condition it isn't super useful otherwise. And it might confuse people who
don't expect the transactional instructions to be used at all because their cpu
doesn't natively support them.
As long as we don't have a more flexible mechanism for selecting the cpuid bits
to support maybe this one should default to off in all cases?
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