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x2apic in qemu
https://forum.osdev.org/viewtopic.php?f=1&t=31146		 Page 1 of 1
Author:  bzt [ Sun Jan 01, 2017 5:32 pm ]
Post subject:  x2apic in qemu
Has anybody tried to emulate x2apic with qemu? I can't get it working.

With TCG, I got this:
Code:
qemu-system-x86_64 -m 32 -d guest_errors -hda bin/disk.dd -cpu IvyBridge,+x2apic -monitor stdio
QEMU 2.8.50 monitor - type 'help' for more information
(qemu) warning: TCG doesn't support requested feature: CPUID.01H:ECX.x2apic [bit 21]

My host cpu supports it, so I've tried KVM:
Code:
qemu-system-x86_64 -m 32 -d guest_errors -hda bin/disk.dd -enable-kvm -cpu host,+x2apic -monitor stdio
QEMU 2.8.50 monitor - type 'help' for more information
(qemu)

No errors this time, but the guest's eax=1 cpuid returns ecx 21 bit clear, no matter what I do.

I've checked qemu's source, and I've found a stupid patch that silently disables x2apic when irqchip is not used, so I've tried
Code:
qemu-system-x86_64 -m 32 -d guest_errors -hda bin/disk.dd -enable-kvm -machine kernel-irqchip=off -cpu host,+x2apic -monitor stdio
QEMU 2.8.50 monitor - type 'help' for more information
(qemu) warning: host doesn't support requested feature: CPUID.01H:ECX.x2apic [bit 21]

which failed as expected. Now with irqchip:
Code:
qemu-system-x86_64 -m 32 -d guest_errors -hda bin/disk.dd -enable-kvm -machine kernel-irqchip=on -cpu host,+x2apic -monitor stdio
QEMU 2.8.50 monitor - type 'help' for more information
(qemu)

No errors, but guest cpuid reports no x2apic...

I'm not sure where else to check if kernel-irqchip is used at all, all I've got is irqbypass:
Code:
# lsmod | grep kvm
kvm_intel             188416  0
kvm                   516096  1 kvm_intel
irqbypass              16384  1 kvm


Any help would be appreciated, I'd really wanna use x2apic!
Author:  SukantPal [ Sun Oct 01, 2017 9:06 am ]
Post subject:  Re: x2apic in qemu
I have also tried to use the X2APIC feature for IA32 architecture. But, QEMU simply doesn't support this feature right now as I have seen. Even, the QEMU message says that X2APIC feature is not supported.
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