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[librm] Speed up real-to-protected mode transition under KVM

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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>
This commit is contained in:
Michael Brown
2014-05-02 13:18:55 +01:00
parent 5a08b63cb7
commit c64747db50
3 changed files with 22 additions and 5 deletions
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5
src/arch/i386/prefix/libprefix.S
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@@ -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 */