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

[Maxim Levitsky]

On Wed, 2021-05-05 at 10:39 +0200, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes:
> 
> > 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@xxxxxxxxxx>
> > > ---
> > >  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?
> > 
> 
> Debug print could've probably stayed or could've been dropped
> completely -- I don't really believe it's going to help
> anyone. Debugging such issues without instrumentation/tracing seems to
> be hard-to-impossible...
> 
> > >  
> > >  	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.
> > 
> 
> To restore the status quo, yes.
> 
> > >  
> > >  	/* 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)
> > 
> 
> It would be great to compare on Intel to be 100% sure the issue is eVMCS
> related, Hyper-V may be behaving quite differently on AMD.
Hi!

I tested this on my Intel machine with and without eVMCS, without changing
any other parameters, running the same VM from a snapshot.

As I said without eVMCS the test survived overnight stress of ~1800 migrations.
With eVMCs, it fails pretty much on first try. 
With those patches, it fails after about 20 iterations.

Best regards,
	Maxim Levitsky

> 
> > 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.
> 
> Thanks for testing! I'll try to come up with a selftest for this issue,
> maybe it'll help us discovering others)
>