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

Maxim Levitsky
2021-05-05T08:22:31Z

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