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A bus lock is acquired either through split locked access to writeback (WB)
memory or by using locks to uncacheable (UC) memory. This is typically > 1000
cycles slower than atomic operation within a cache
line. It also disrupts performance on other cores (which must wait for the bus
lock to be released before their memory operations).
Bus lock debug exception is a sub-feature of bus lock detection. It is
an ability to notify the kernel by an #DB trap after the instruction
acquires a bus lock when CPL>0. This allows the kernel to enforce user
application throttling or mitigations.
A logical processor can be configured to generate a debug exception
(#DB) following acquisition of a bus lock.
Software enables those debug exceptions by setting bit 2 of the
IA32_DEBUGCTL MSR. The CPU enumerates this feature using CPUID.(EAX=7,
ECX=0).ECX[24].
A debug exception due to acquistion of a bus lock is reported as a trap
following execution of the instruction acquiring the bus lock. The
processor identifies such debug exceptions using bit 11 of DR6: delivery
of a bus-lock #DB clears DR6[11].
Linux 5.13
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
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KVM Bus Lock Debug Exception
https://bugs.launchpad.net/bugs/1954469
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