On Tue, Aug 06, 2013 at 10:18:17AM +0200, Jan Kiszka wrote:
> 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 <[email protected]> wrote:
> >>>>>>> From: Nadav Har'El <[email protected]>
> >>>>>>>
> >>>>>>> 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 <[email protected]>
> >>>>>>> Signed-off-by: Nadav Har'El <[email protected]>
> >>>>>>> Signed-off-by: Jun Nakajima <[email protected]>
> >>>>>>> Signed-off-by: Xinhao Xu <[email protected]>
> >>>>>>> Signed-off-by: Yang Zhang <[email protected]>
> >>>>>>> Signed-off-by: Gleb Natapov <[email protected]>
> >>>>>>> ---
> >>>>>>> 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.
>
We do not shadow all VMCS fields, only some of them (see
shadow_read_write_fields array). HOST/GUEST_IA32_EFER is not the one we
shadow.
--
Gleb.
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