Re: [PATCH v1 05/23] KVM: VMX: Initialize FRED VM entry/exit controls in vmcs_config

[Sean Christopherson]

On Thu, Nov 09, 2023, Chao Gao wrote:
> On Wed, Nov 08, 2023 at 10:29:45AM -0800, Xin Li wrote:
> >Setup the global vmcs_config for FRED:
> >1) Add VM_ENTRY_LOAD_IA32_FRED to KVM_OPTIONAL_VMX_VM_ENTRY_CONTROLS to
> >   have a FRED CPU load guest FRED MSRs from VMCS upon VM entry.
> >2) Add SECONDARY_VM_EXIT_SAVE_IA32_FRED to
> >   KVM_OPTIONAL_VMX_SECONDARY_VM_EXIT_CONTROLS to have a FRED CPU save
> >   guest FRED MSRs to VMCS during VM exit.
> >3) add SECONDARY_VM_EXIT_LOAD_IA32_FRED to
> >   KVM_OPTIONAL_VMX_SECONDARY_VM_EXIT_CONTROLS to have a FRED CPU load
> >   host FRED MSRs from VMCS during VM exit.
> >
> >Also add sanity checks to make sure FRED VM entry/exit controls can be
> >set on a FRED CPU.
> >
> >Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
> >Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
> >---
> > arch/x86/include/asm/vmx.h |  3 +++
> > arch/x86/kvm/vmx/vmx.c     | 19 ++++++++++++++++++-
> > arch/x86/kvm/vmx/vmx.h     |  7 +++++--
> > 3 files changed, 26 insertions(+), 3 deletions(-)
> >
> >diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> >index 4d4177ec802c..41796a733bc9 100644
> >--- a/arch/x86/include/asm/vmx.h
> >+++ b/arch/x86/include/asm/vmx.h
> >@@ -106,6 +106,8 @@
> > #define VM_EXIT_PT_CONCEAL_PIP			0x01000000
> > #define VM_EXIT_CLEAR_IA32_RTIT_CTL		0x02000000
> > #define VM_EXIT_ACTIVATE_SECONDARY_CONTROLS	0x80000000
> >+#define SECONDARY_VM_EXIT_SAVE_IA32_FRED	0x00000001
> >+#define SECONDARY_VM_EXIT_LOAD_IA32_FRED	0x00000002
> > 
> > #define VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR	0x00036dff
> > 
> >@@ -119,6 +121,7 @@
> > #define VM_ENTRY_LOAD_BNDCFGS                   0x00010000
> > #define VM_ENTRY_PT_CONCEAL_PIP			0x00020000
> > #define VM_ENTRY_LOAD_IA32_RTIT_CTL		0x00040000
> >+#define VM_ENTRY_LOAD_IA32_FRED			0x00800000
> > 
> > #define VM_ENTRY_ALWAYSON_WITHOUT_TRUE_MSR	0x000011ff
> > 
> >diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >index df769207cbe0..9186f41974ab 100644
> >--- a/arch/x86/kvm/vmx/vmx.c
> >+++ b/arch/x86/kvm/vmx/vmx.c
> >@@ -2694,10 +2694,27 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> > 		_vmexit_control &= ~x_ctrl;
> > 	}
> > 
> >-	if (_vmexit_control & VM_EXIT_ACTIVATE_SECONDARY_CONTROLS)
> >+	if (_vmexit_control & VM_EXIT_ACTIVATE_SECONDARY_CONTROLS) {
> > 		_secondary_vmexit_control =
> > 			adjust_vmx_controls64(KVM_OPTIONAL_VMX_SECONDARY_VM_EXIT_CONTROLS,
> > 					      MSR_IA32_VMX_EXIT_CTLS2);
> >+		if (cpu_feature_enabled(X86_FEATURE_FRED) &&
> >+		    !(_secondary_vmexit_control & SECONDARY_VM_EXIT_SAVE_IA32_FRED &&
> >+		      _secondary_vmexit_control & SECONDARY_VM_EXIT_LOAD_IA32_FRED)) {
> >+			pr_warn_once("FRED enabled but no VMX VM-Exit {SAVE,LOAD}_IA32_FRED controls: %llx\n",
> >+				     _secondary_vmexit_control);
> 
> if there is no VM_EXIT_ACTIVATE_SECONDARY_CONTROLS, shouldn't we also emit this
> warning?
> 
> >+			if (error_on_inconsistent_vmcs_config)
> >+				return -EIO;
> >+		}
> >+	}
> >+
> >+	if (cpu_feature_enabled(X86_FEATURE_FRED) &&
> >+	    !(_vmentry_control & VM_ENTRY_LOAD_IA32_FRED)) {
> >+		pr_warn_once("FRED enabled but no VMX VM-Entry LOAD_IA32_FRED control: %x\n",
> >+			     _vmentry_control);
> 
> Can we just hide FRED from guests like what KVM does for other features which
> have similar dependencies? see vmx_set_cpu_caps().

Both of these warnings should simply be dropped.  The error_on_inconsistent_vmcs_config
stuff is for inconsistencies within the allowed VMCS fields.  Having a feature
that is supported in bare metal but not virtualized is perfectly legal, if
uncommon.

What *is* needed is for KVM to refuse to virtualize FRED if the entry/exit controls
aren't consistent.  E.g. if at least one control is present, and at least one
control is missing.   I.e. KVM needs a version of vmcs_entry_exit_pairs that can
deal with SECONDAY_VM_EXIT controls.  I'll circle back to this when I give the
series a proper review, which is going to be 3+ weeks.