On 21.08.2014 12:28, Valentine Sinitsyn wrote:
KVM seems to work properly (no weird exceptions injected), although my
code now freezes (quick look on the trace suggests it's looping reading
APIC). Not sure whose bug is it, will look further.
Looks like the problem is that if nested page tables maps some GPA to
0xfee00000 HPA, it's really mapped to this HPA, and not intercepted with
KVM's virtual LAPIC implementation. Consider the following trace:
qemu-system-x86-344 [000] 644.974072: kvm_entry: vcpu 0
qemu-system-x86-344 [000] 644.974075: kvm_exit: reason npf rip 0xffffffff8104e883 info 10000000d fee000f0
qemu-system-x86-344 [000] 644.974075: kvm_page_fault: address fee000f0 error_code d
qemu-system-x86-344 [000] 644.974077: kvm_emulate_insn: 0:ffffffff8104e883:8b 87 00 b0 5f ff (prot64)
qemu-system-x86-344 [000] 644.974078: kvm_apic: apic_read APIC_SPIV = 0xf
qemu-system-x86-344 [000] 644.974079: kvm_mmio: mmio read len 4 gpa 0xfee000f0 val 0x72007200000000f
qemu-system-x86-344 [000] 644.974081: kvm_entry: vcpu 0
Here, I set up NPT so that any access to 0xfee00000 nested guest
physical address cause VM exit. Then, my code writes or reads register
that is mapped to 0xfee00000 KVM's GPA. kvm_apic is called, and
everything works as expected.
However, if I set up NTP to make 0xfee00000 nested guest physical
address reads don't cause nested VM exit (by simply clearing U/S flag in
the NPTE), I get:
qemu-system-x86-1066 [003] 1105.864286: kvm_exit: reason npf rip 0xffffffff8104eaa4 info 10000000f fee00310
qemu-system-x86-1066 [003] 1105.864287: kvm_nested_vmexit: rip: 0xffffffff8104eaa4 reason: npf ext_inf1: 0x000000010000000f ext_inf2: 0x00000000fee00310 ext_int: 0x00000000 ext_int_err: 0x00000000
qemu-system-x86-1066 [003] 1105.864287: kvm_page_fault: address fee00310 error_code f
qemu-system-x86-1064 [001] 1105.864288: kvm_exit: reason npf rip 0xffffffff8104e876 info 10000000f fee000b0
qemu-system-x86-1066 [003] 1105.864289: kvm_emulate_insn: 0:ffffffff8104eaa4:89 14 25 10 b3 5f ff (prot64)
qemu-system-x86-1064 [001] 1105.864289: kvm_nested_vmexit: rip: 0xffffffff8104e876 reason: npf ext_inf1: 0x000000010000000f ext_inf2: 0x00000000fee000b0 ext_int: 0x00000000 ext_int_err: 0x00000000
qemu-system-x86-1064 [001] 1105.864289: kvm_page_fault: address fee000b0 error_code f
qemu-system-x86-1064 [001] 1105.864291: kvm_emulate_insn: 0:ffffffff8104e876:89 b7 00 b0 5f ff (prot64)
qemu-system-x86-1066 [003] 1105.864292: kvm_inj_exception: e (0x2)
qemu-system-x86-1066 [003] 1105.864293: kvm_entry: vcpu 3
qemu-system-x86-1064 [001] 1105.864294: kvm_inj_exception: e (0x2)
qemu-system-x86-1064 [001] 1105.864295: kvm_entry: vcpu 1
No kvm_apic: after NPTs are set up, no page faults caused by register
read (error_code: d), to trap and emulate APIC access.
So I'm returning to my original question: is this an intended behavior
of KVM that APIC access on nested page tables level are not trapped, or
is this a bug?
Valentine
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