On 2013-08-06 10:00, Gleb Natapov wrote: > On Tue, Aug 06, 2013 at 09:55:09AM +0200, Jan Kiszka wrote: >> On 2013-08-06 09:51, Gleb Natapov wrote: >>> On Tue, Aug 06, 2013 at 09:47:23AM +0200, Jan Kiszka wrote: >>>> On 2013-08-05 13:40, Gleb Natapov wrote: >>>>> On Mon, Aug 05, 2013 at 07:27:33PM +0800, Arthur Chunqi Li wrote: >>>>>> On Mon, Aug 5, 2013 at 4:07 PM, Gleb Natapov <gleb@xxxxxxxxxx> wrote: >>>>>>> From: Nadav Har'El <nyh@xxxxxxxxxx> >>>>>>> >>>>>>> Recent KVM, since http://kerneltrap.org/mailarchive/linux-kvm/2010/5/2/6261577 >>>>>>> switch the EFER MSR when EPT is used and the host and guest have different >>>>>>> NX bits. So if we add support for nested EPT (L1 guest using EPT to run L2) >>>>>>> and want to be able to run recent KVM as L1, we need to allow L1 to use this >>>>>>> EFER switching feature. >>>>>>> >>>>>>> To do this EFER switching, KVM uses VM_ENTRY/EXIT_LOAD_IA32_EFER if available, >>>>>>> and if it isn't, it uses the generic VM_ENTRY/EXIT_MSR_LOAD. This patch adds >>>>>>> support for the former (the latter is still unsupported). >>>>>>> >>>>>>> Nested entry and exit emulation (prepare_vmcs_02 and load_vmcs12_host_state, >>>>>>> respectively) already handled VM_ENTRY/EXIT_LOAD_IA32_EFER correctly. So all >>>>>>> that's left to do in this patch is to properly advertise this feature to L1. >>>>>>> >>>>>>> Note that vmcs12's VM_ENTRY/EXIT_LOAD_IA32_EFER are emulated by L0, by using >>>>>>> vmx_set_efer (which itself sets one of several vmcs02 fields), so we always >>>>>>> support this feature, regardless of whether the host supports it. >>>>>>> >>>>>>> Reviewed-by: Orit Wasserman <owasserm@xxxxxxxxxx> >>>>>>> Signed-off-by: Nadav Har'El <nyh@xxxxxxxxxx> >>>>>>> Signed-off-by: Jun Nakajima <jun.nakajima@xxxxxxxxx> >>>>>>> Signed-off-by: Xinhao Xu <xinhao.xu@xxxxxxxxx> >>>>>>> Signed-off-by: Yang Zhang <yang.z.zhang@xxxxxxxxx> >>>>>>> Signed-off-by: Gleb Natapov <gleb@xxxxxxxxxx> >>>>>>> --- >>>>>>> arch/x86/kvm/vmx.c | 23 ++++++++++++++++------- >>>>>>> 1 file changed, 16 insertions(+), 7 deletions(-) >>>>>>> >>>>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >>>>>>> index e999dc7..27efa6a 100644 >>>>>>> --- a/arch/x86/kvm/vmx.c >>>>>>> +++ b/arch/x86/kvm/vmx.c >>>>>>> @@ -2198,7 +2198,8 @@ static __init void nested_vmx_setup_ctls_msrs(void) >>>>>>> #else >>>>>>> nested_vmx_exit_ctls_high = 0; >>>>>>> #endif >>>>>>> - nested_vmx_exit_ctls_high |= VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR; >>>>>>> + nested_vmx_exit_ctls_high |= (VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR | >>>>>>> + VM_EXIT_LOAD_IA32_EFER); >>>>>> Gleb, why we don't need to check whether host supports >>>>>> VM_EXIT_LOAD_IA32_EFER here, as what you noted in my >>>>>> VM_EXIT_LOAD_IA32_PAT patch? >>>>> Nested VMX completely emulates the capability. >>>> >>>> No, it doesn't. The values for host/guest are handled over via the >>>> corresponding VMCS fields, physically, even though the actual loading is >>>> emulated then. So we must not expose this feature unconditionally. >>> Can you show me the code where it happens? >> >> When the guest writes to HOST/GUEST_IA32_EFER, we will store this in the >> vmcs that will then become the active one on next L1/L2 entry, no? >> > Guest writes are stored in vmcs12 which is not HW vmcs, just a format > kvm uses internally (see vmcs12_write_any). During guest entry vmcs02 > is constructed from vmcs12 (see prepare_vmcs02) and this function does > not access HOST/GUEST_IA32_EFER directly, it uses vmx_set_efer instead > which takes care of things. Same function access GUEST_IA32_PAT directly > though. OK, right. Is it also safe to write to any field of a shadow VMCS? That's currently hypothetical, but what if the host supports shadowing but not EFER loading? I don't see a technical reason but also no clear statement in the SDM regarding this scenario. Jan
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