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| author | Michael Brown | 2014-05-02 14:18:55 +0200 |
|---|---|---|
| committer | Michael Brown | 2014-05-02 16:23:21 +0200 |
| commit | c64747db5040d860c9c77f2673c4cce287249bbd (patch) | |
| tree | 1c9ddbd0516a5240916bcd79eae9ca4652c13ffa /src/arch/i386/prefix/libprefix.S | |
| parent | [librm] Speed up protected-to-real mode transition under KVM (diff) | |
| download | ipxe-c64747db5040d860c9c77f2673c4cce287249bbd.tar.gz ipxe-c64747db5040d860c9c77f2673c4cce287249bbd.tar.xz ipxe-c64747db5040d860c9c77f2673c4cce287249bbd.zip | |
[librm] Speed up real-to-protected mode transition under KVM
Ensure that all segment registers have zero in the low two bits before
transitioning to protected mode. This allows the CPU state to
immediately be deemed to be "valid", and eliminates the need for any
further emulated instructions.
Load the protected-mode interrupt descriptor table after switching to
protected mode, since this avoids triggering an EXCEPTION_NMI and
corresponding VM exit.
This reduces the time taken by real_to_prot under KVM by around 50%.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Diffstat (limited to 'src/arch/i386/prefix/libprefix.S')
| -rw-r--r-- | src/arch/i386/prefix/libprefix.S | 5 |
1 files changed, 4 insertions, 1 deletions
diff --git a/src/arch/i386/prefix/libprefix.S b/src/arch/i386/prefix/libprefix.S index 197a86bc..3aee415f 100644 --- a/src/arch/i386/prefix/libprefix.S +++ b/src/arch/i386/prefix/libprefix.S @@ -522,8 +522,11 @@ alloc_basemem: subw $_data16_memsz_pgh, %ax pushw %ax - /* Calculate .text16 segment address */ + /* Calculate .text16 segment address. Round down to ensure + * low bits are zero, to speed up mode transitions under KVM. + */ subw $_text16_memsz_pgh, %ax + andb $~0x03, %al pushw %ax /* Update FBMS */ |