Specification Update 29
BV27. Fault Not Reported When Setting Reserved Bits of Intel® VT-d Queued Invalidation Descriptors
Problem: Reserved bits in the Queued Invalidation descriptors of Intel VT-d (Virtualization
Technology for Directed I/O) are expected to be zero, meaning that software must
program them as zero while the processor checks if they are not zero. Upon detection
of a non-zero bit in a reserved field an Intel VT-d fault should be recorded. Due to this
erratum the processor does not check reserved bit values for Queued Invalidation
descriptors.
Implication: Due to this erratum, faults will not be reported when writing to reserved bits of Intel
VT-d Queued Invalidation Descriptors.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.
BV28. FP Data Operand Pointer May Be Incorrectly Calculated After an FP Access Which Wraps a 4-Gbyte Boundary in Code That Uses 32-Bit Address Size in 64-bit Mode
Problem: The FP (Floating Point) Data Operand Pointer is the effective address of the operand
associated with the last non-control FP instruction executed by the processor. If an 80-
bit FP access (load or store) uses a 32-bit address size in 64-bit mode and the memory
access wraps a 4-Gbyte boundary and the FP environment is subsequently saved, the
value contained in the FP Data Operand Pointer may be incorrect.
Implication: Due to this erratum, the FP Data Operand Pointer may be incorrect. Wrapping an 80-bit
FP load around a 4-Gbyte boundary in this way is not a normal programming practice.
Intel has not observed this erratum with any commercially available software.
Workaround: If the FP Data Operand Pointer is used in a 64-bit operating system which may run code
accessing 32-bit addresses, care must be taken to ensure that no 80-bit FP accesses
are wrapped around a 4-Gbyte boundary.
Status: For the steppings affected, see the Summary Tables of Changes.
BV29. VMREAD/VMWRITE Instruction May Not Fail When Accessing an Unsupported Field in VMCS
Problem: The Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 2B states
that execution of VMREAD or VMWRITE should fail if the value of the instruction’s
register source operand corresponds to an unsupported field in the VMCS (Virtual
Machine Control Structure). The correct operation is that the logical processor will set
the ZF (Zero Flag), write 0CH into the VM-instruction error field and for VMREAD leave
the instruction’s destination operand unmodified. Due to this erratum, the instruction
may instead clear the ZF, leave the VM-instruction error field unmodified and for
VMREAD modify the contents of its destination operand.
Implication: Accessin g an unsu pported field in V MCS will fail to p roperly r eport an error. In addi tion,
VMREAD from an unsupported VMCS field may unexpectedly change its destination
operand. Intel has not observed this erratum with any commercially available software.
Workaround: Software should avoid accessing unsupported fields in a VMCS.
Status: For the steppings affected, see the Summary Tables of Changes.