Re: [PATCH 1/4] KVM: nVMX: Always make an attempt to map eVMCS after migration

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2021-05-03 at 17:08 +0200, Vitaly Kuznetsov wrote:
> When enlightened VMCS is in use and nested state is migrated with
> vmx_get_nested_state()/vmx_set_nested_state() KVM can't map evmcs
> page right away: evmcs gpa is not 'struct kvm_vmx_nested_state_hdr'
> and we can't read it from VP assist page because userspace may decide
> to restore HV_X64_MSR_VP_ASSIST_PAGE after restoring nested state
> (and QEMU, for example, does exactly that). To make sure eVMCS is
> mapped /vmx_set_nested_state() raises KVM_REQ_GET_NESTED_STATE_PAGES
> request.
> 
> Commit f2c7ef3ba955 ("KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES
> on nested vmexit") added KVM_REQ_GET_NESTED_STATE_PAGES clearing to
> nested_vmx_vmexit() to make sure MSR permission bitmap is not switched
> when an immediate exit from L2 to L1 happens right after migration (caused
> by a pending event, for example). Unfortunately, in the exact same
> situation we still need to have eVMCS mapped so
> nested_sync_vmcs12_to_shadow() reflects changes in VMCS12 to eVMCS.
> 
> As a band-aid, restore nested_get_evmcs_page() when clearing
> KVM_REQ_GET_NESTED_STATE_PAGES in nested_vmx_vmexit(). The 'fix' is far
> from being ideal as we can't easily propagate possible failures and even if
> we could, this is most likely already too late to do so. The whole
> 'KVM_REQ_GET_NESTED_STATE_PAGES' idea for mapping eVMCS after migration
> seems to be fragile as we diverge too much from the 'native' path when
> vmptr loading happens on vmx_set_nested_state().
> 
> Fixes: f2c7ef3ba955 ("KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES on nested vmexit")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 1e069aac7410..2febb1dd68e8 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3098,15 +3098,8 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
>  			nested_vmx_handle_enlightened_vmptrld(vcpu, false);
>  
>  		if (evmptrld_status == EVMPTRLD_VMFAIL ||
> -		    evmptrld_status == EVMPTRLD_ERROR) {
> -			pr_debug_ratelimited("%s: enlightened vmptrld failed\n",
> -					     __func__);
> -			vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> -			vcpu->run->internal.suberror =
> -				KVM_INTERNAL_ERROR_EMULATION;
> -			vcpu->run->internal.ndata = 0;
> +		    evmptrld_status == EVMPTRLD_ERROR)
>  			return false;
> -		}
>  	}
>  
>  	return true;
> @@ -3194,8 +3187,16 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
>  
>  static bool vmx_get_nested_state_pages(struct kvm_vcpu *vcpu)
>  {
> -	if (!nested_get_evmcs_page(vcpu))
> +	if (!nested_get_evmcs_page(vcpu)) {
> +		pr_debug_ratelimited("%s: enlightened vmptrld failed\n",
> +				     __func__);
> +		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> +		vcpu->run->internal.suberror =
> +			KVM_INTERNAL_ERROR_EMULATION;
> +		vcpu->run->internal.ndata = 0;
> +
>  		return false;
> +	}

Hi!

Any reason to move the debug prints out of nested_get_evmcs_page?


>  
>  	if (is_guest_mode(vcpu) && !nested_get_vmcs12_pages(vcpu))
>  		return false;
> @@ -4422,7 +4423,15 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
>  	/* trying to cancel vmlaunch/vmresume is a bug */
>  	WARN_ON_ONCE(vmx->nested.nested_run_pending);
>  
> -	kvm_clear_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
> +	if (kvm_check_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu)) {
> +		/*
> +		 * KVM_REQ_GET_NESTED_STATE_PAGES is also used to map
> +		 * Enlightened VMCS after migration and we still need to
> +		 * do that when something is forcing L2->L1 exit prior to
> +		 * the first L2 run.
> +		 */
> +		(void)nested_get_evmcs_page(vcpu);
> +	}
Yes this is a band-aid, but it has to be done I agree.

>  
>  	/* Service the TLB flush request for L2 before switching to L1. */
>  	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))




I also tested this and it survives a bit better (used to crash instantly
after a single migration cycle, but the guest still crashes after around ~20 iterations of my 
regular nested migration test).

Blues screen shows that stop code is HYPERVISOR ERROR and nothing else.

I tested both this patch alone and all 4 patches.

Without evmcs, the same VM with same host kernel and qemu survived an overnight
test and passed about 1800 migration iterations.
(my synthetic migration test doesn't yet work on Intel, I need to investigate why)

For reference this is the VM that you gave me to test, kvm/queue kernel,
with merged mainline in it,
and mostly latest qemu (updated about a week ago or so)

qemu: 3791642c8d60029adf9b00bcb4e34d7d8a1aea4d
kernel: 9f242010c3b46e63bc62f08fff42cef992d3801b and
        then merge v5.12 from mainline.

Best regards,
	Maxim Levitsky