[Devel] [PATCH RHEL7 COMMIT] ms/x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested

[Konstantin Khorenko]

Makes sense to release it as a ReadyKernel patch?

Or the code is executed too early, so modules won't be loaded yet?

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 02/14/2018 01:43 PM, Konstantin Khorenko wrote:
> The commit is pushed to "branch-rh7-3.10.0-693.17.1.vz7.43.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
> after rh7-3.10.0-693.17.1.vz7.43.4
> ------>
> commit 988f48689f68fe80186f2ec994e4f135769c1d1b
> Author: Vitaly Kuznetsov <vkuznets at redhat.com>
> Date:   Wed Feb 14 13:43:38 2018 +0300
>
>     ms/x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested
>
>     I was investigating an issue with seabios >= 1.10 which stopped working
>     for nested KVM on Hyper-V. The problem appears to be in
>     handle_ept_violation() function: when we do fast mmio we need to skip
>     the instruction so we do kvm_skip_emulated_instruction(). This, however,
>     depends on VM_EXIT_INSTRUCTION_LEN field being set correctly in VMCS.
>     However, this is not the case.
>
>     Intel's manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set when
>     EPT MISCONFIG occurs. While on real hardware it was observed to be set,
>     some hypervisors follow the spec and don't set it; we end up advancing
>     IP with some random value.
>
>     I checked with Microsoft and they confirmed they don't fill
>     VM_EXIT_INSTRUCTION_LEN on EPT MISCONFIG.
>
>     Fix the issue by doing instruction skip through emulator when running
>     nested.
>
>     Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
>     Suggested-by: Radim Krčmář <rkrcmar at redhat.com>
>     Suggested-by: Paolo Bonzini <pbonzini at redhat.com>
>     Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
>     Acked-by: Michael S. Tsirkin <mst at redhat.com>
>     Signed-off-by: Radim Krčmář <rkrcmar at redhat.com>
>
>     [rkagan: the problem pertains to ESXi as well]
>     (cherry picked from commit d391f1207067268261add0485f0f34503539c5b0)
>     [rkagan: adjusted for vz7.7]
>
>     https://jira.sw.ru/browse/PSBM-81462
>     Signed-off-by: Roman Kagan <rkagan at virtuozzo.com>
> ---
>  arch/x86/kvm/vmx.c | 18 ++++++++++++++++--
>  arch/x86/kvm/x86.c |  3 ++-
>  2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index d94946a07de6..8ab2b3d42adc 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5934,9 +5934,23 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
>
>  	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
>  	if (!kvm_io_bus_write(vcpu->kvm, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
> -		skip_emulated_instruction(vcpu);
>  		trace_kvm_fast_mmio(gpa);
> -		return 1;
> +		/*
> +		 * Doing kvm_skip_emulated_instruction() depends on undefined
> +		 * behavior: Intel's manual doesn't mandate
> +		 * VM_EXIT_INSTRUCTION_LEN to be set in VMCS when EPT MISCONFIG
> +		 * occurs and while on real hardware it was observed to be set,
> +		 * other hypervisors (namely Hyper-V) don't set it, we end up
> +		 * advancing IP with some random value. Disable fast mmio when
> +		 * running nested and keep it for real hardware in hope that
> +		 * VM_EXIT_INSTRUCTION_LEN will always be set correctly.
> +		 */
> +		if (!static_cpu_has(X86_FEATURE_HYPERVISOR)) {
> +			skip_emulated_instruction(vcpu);
> +			return 1;
> +		} else
> +			return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP,
> +						       NULL, 0) == EMULATE_DONE;
>  	}
>
>  	ret = handle_mmio_page_fault(vcpu, gpa, true);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c3103bdd2d37..ddb3c2529ca8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5503,7 +5503,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>  		 * handle watchpoints yet, those would be handled in
>  		 * the emulate_ops.
>  		 */
> -		if (kvm_vcpu_check_breakpoint(vcpu, &r))
> +		if (!(emulation_type & EMULTYPE_SKIP) &&
> +		    kvm_vcpu_check_breakpoint(vcpu, &r))
>  			return r;
>
>  		ctxt->interruptibility = 0;
> .
>