Commit 6d849191 authored by Oliver Upton's avatar Oliver Upton Committed by Paolo Bonzini
Browse files

KVM: x86: Introduce KVM_CAP_DISABLE_QUIRKS2 



KVM_CAP_DISABLE_QUIRKS is irrevocably broken. The capability does not
advertise the set of quirks which may be disabled to userspace, so it is
impossible to predict the behavior of KVM. Worse yet,
KVM_CAP_DISABLE_QUIRKS will tolerate any value for cap->args[0], meaning
it fails to reject attempts to set invalid quirk bits.

The only valid workaround for the quirky quirks API is to add a new CAP.
Actually advertise the set of quirks that can be disabled to userspace
so it can predict KVM's behavior. Reject values for cap->args[0] that
contain invalid bits.

Finally, add documentation for the new capability and describe the
existing quirks.

Signed-off-by: default avatarOliver Upton <oupton@google.com>
Message-Id: <20220301060351.442881-5-oupton@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 5e17b2ee

Documentation/virt/kvm/api.rst

+50 −0
Original line number Diff line number Diff line
@@ -7083,6 +7083,56 @@ resource that is controlled with the H_SET_MODE hypercall. 
This capability allows a guest kernel to use a better-performance mode for

handling interrupts and system calls.



7.31 KVM_CAP_DISABLE_QUIRKS2

----------------------------



:Capability: KVM_CAP_DISABLE_QUIRKS2

:Parameters: args[0] - set of KVM quirks to disable

:Architectures: x86

:Type: vm



This capability, if enabled, will cause KVM to disable some behavior

quirks.



Calling KVM_CHECK_EXTENSION for this capability returns a bitmask of

quirks that can be disabled in KVM.



The argument to KVM_ENABLE_CAP for this capability is a bitmask of

quirks to disable, and must be a subset of the bitmask returned by

KVM_CHECK_EXTENSION.



The valid bits in cap.args[0] are:



=================================== ============================================

 KVM_X86_QUIRK_LINT0_REENABLED      By default, the reset value for the LVT

                                    LINT0 register is 0x700 (APIC_MODE_EXTINT).

                                    When this quirk is disabled, the reset value

                                    is 0x10000 (APIC_LVT_MASKED).



 KVM_X86_QUIRK_CD_NW_CLEARED        By default, KVM clears CR0.CD and CR0.NW.

                                    When this quirk is disabled, KVM does not

                                    change the value of CR0.CD and CR0.NW.



 KVM_X86_QUIRK_LAPIC_MMIO_HOLE      By default, the MMIO LAPIC interface is

                                    available even when configured for x2APIC

                                    mode. When this quirk is disabled, KVM

                                    disables the MMIO LAPIC interface if the

                                    LAPIC is in x2APIC mode.



 KVM_X86_QUIRK_OUT_7E_INC_RIP       By default, KVM pre-increments %rip before

                                    exiting to userspace for an OUT instruction

                                    to port 0x7e. When this quirk is disabled,

                                    KVM does not pre-increment %rip before

                                    exiting to userspace.



 KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT When this quirk is disabled, KVM sets

                                    CPUID.01H:ECX[bit 3] (MONITOR/MWAIT) if

                                    IA32_MISC_ENABLE[bit 18] (MWAIT) is set.

                                    Additionally, when this quirk is disabled,

                                    KVM clears CPUID.01H:ECX[bit 3] if

                                    IA32_MISC_ENABLE[bit 18] is cleared.

=================================== ============================================



8. Other capabilities.

======================

arch/x86/include/asm/kvm_host.h

+7 −0
Original line number Diff line number Diff line
@@ -1966,4 +1966,11 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages); 
#define KVM_CLOCK_VALID_FLAGS						\

	(KVM_CLOCK_TSC_STABLE | KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC)



#define KVM_X86_VALID_QUIRKS			\

	(KVM_X86_QUIRK_LINT0_REENABLED |	\

	 KVM_X86_QUIRK_CD_NW_CLEARED |		\

	 KVM_X86_QUIRK_LAPIC_MMIO_HOLE |	\

	 KVM_X86_QUIRK_OUT_7E_INC_RIP |		\

	 KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)



#endif /* _ASM_X86_KVM_HOST_H */

arch/x86/kvm/x86.c

+8 −0
Original line number Diff line number Diff line
@@ -4363,6 +4363,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) 
		r = enable_pmu ? KVM_CAP_PMU_VALID_MASK : 0;

		break;

	}

	case KVM_CAP_DISABLE_QUIRKS2:

		r = KVM_X86_VALID_QUIRKS;

		break;

	default:

		break;

	}
@@ -5909,6 +5912,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, 
		return -EINVAL;



	switch (cap->cap) {

	case KVM_CAP_DISABLE_QUIRKS2:

		r = -EINVAL;

		if (cap->args[0] & ~KVM_X86_VALID_QUIRKS)

			break;

		fallthrough;

	case KVM_CAP_DISABLE_QUIRKS:

		kvm->arch.disabled_quirks = cap->args[0];

		r = 0;

include/uapi/linux/kvm.h

+1 −0
Original line number Diff line number Diff line
@@ -1143,6 +1143,7 @@ struct kvm_ppc_resize_hpt { 
#define KVM_CAP_PPC_AIL_MODE_3 210

#define KVM_CAP_S390_MEM_OP_EXTENSION 211

#define KVM_CAP_PMU_CAPABILITY 212

#define KVM_CAP_DISABLE_QUIRKS2 213



#ifdef KVM_CAP_IRQ_ROUTING