ba927d042e
The Arm architecture specifies a number of ID registers that are
characterized as comprising a set of 4-bit ID fields. Each ID field
identifies the presence, and possibly the level of support for, a
particular feature in an implementation of the architecture. [1]
For most of the ID fields, there is a minimum presence value, equal to
or higher than which means the corresponding CPU feature is implemented.
Hence, we can use the minimum presence value to determine whether a CPU
feature is enabled and enable a CPU feature.
To disable a CPU feature, setting the corresponding ID field to 0x0/0xf
(for unsigned/signed field) seems as a good idea. However, it maybe
lead to some problems. For example, ID_AA64PFR0_EL1.FP is a signed ID
field. ID_AA64PFR0_EL1.FP == 0x0 represents the implementation of FP
(floating-point) and ID_AA64PFR0_EL1.FP == 0x1 represents the
implementation of FPHP (half-precision floating-point). If
ID_AA64PFR0_EL1.FP is set to 0xf when FPHP is disabled (which is also
disable FP), guest kernel maybe stuck. Hence, we add a ni_value (means
not-implemented value) to disable a CPU feature safely.
[1] D13.1.3 Principles of the ID scheme for fields in ID registers in
DDI.0487
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Signed-off-by: Dongxu Sun <sundongxu3@huawei.com>