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Vol. 3 16-29
DEBUGGING, PROFILING BRANCHES AND TIME-STAMP COUNTER
• It is recommended that the buffer size for the BTS buffer and the PEBS buffer be
an integer multiple of the corresponding record sizes.
• The precise event records buffer should be large enough to hold the number of
precise event records that can occur while waiting for the interrupt to be
serviced.
• The DS save area should be in kernel space. It must not be on the same page as
code, to avoid triggering self-modifying code actions.
• There are no memory type restrictions on the buffers, although it is
recommended that the buffers be designated as WB memory type for
performance considerations.
• Either the system must be prevented from entering A20M mode while DS save
area is active, or bit 20 of all addresses within buffer bounds must be 0.
• Pages that contain buffers must be mapped to the same physical addresses for all
processes, such that any change to control register CR3 will not change the DS
addresses.
• The DS save area is expected to used only on systems with an enabled APIC. The
LVT Performance Counter entry in the APCI must be initialized to use an interrupt
gate instead of the trap gate.
16.4.9.3 Setting Up the BTS Buffer
Three flags in the MSR_DEBUGCTLA MSR (see Table 16-4), IA32_DEBUGCTL (see
Figure 16-3), or MSR_DEBUGCTLB (see Figure 16-16) control the generation of
branch records and storing of them in the BTS buffer; these are TR, BTS, and BTINT.
The TR flag enables the generation of BTMs. The BTS flag determines whether the
BTMs are sent out on the system bus (clear) or stored in the BTS buffer (set). BTMs
cannot be simultaneously sent to the system bus and logged in the BTS buffer. The
BTINT flag enables the generation of an interrupt when the BTS buffer is full. When
this flag is clear, the BTS buffer is a circular buffer.
The following procedure describes how to set up a DS Save area to collect branch
records in the BTS buffer:
1. Place values in the BTS buffer base, BTS index, BTS absolute maximum, and BTS
interrupt threshold fields of the DS buffer management area to set up the BTS
buffer in memory.
Table 16-4. IA32_DEBUGCTL Flag Encodings
TR BTS BTINT Description
0 X X Branch trace messages (BTMs) off
1 0 X Generate BTMs
1 1 0 Store BTMs in the BTS buffer, used here as a circular buffer
1 1 1 Store BTMs in the BTS buffer, and generate an interrupt when
the buffer is nearly full