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Subject[PATCH] KVM: VMX: Condition ENCLS-exiting enabling on CPU support for SGX1
Date
Enable ENCLS-exiting (and thus set vmcs.ENCLS_EXITING_BITMAP) only if
the CPU supports SGX1. Per Intel's SDM, all ENCLS leafs #UD if SGX1
is not supported[*], i.e. intercepting ENCLS to inject a #UD is
unnecessary.

Avoiding ENCLS-exiting even when it is reported as supported by the CPU
works around a reported issue where SGX is "hard" disabled after an S3
suspend/resume cycle, i.e. CPUID.0x7.SGX=0 and the VMCS field/control
are enumerated as unsupported. While the root cause of the S3 issue is
unknown, it's definitely _not_ a KVM (or kernel) bug, i.e. this is a
workaround for what is most likely a hardware or firmware issue. As a
bonus side effect, KVM saves a VMWRITE when first preparing vmcs01 and
vmcs02.

Query CPUID directly instead of going through cpu_data() or cpu_has() to
ensure KVM is trapping ENCLS when it's supported in hardware, e.g. even
if X86_FEATURE_SGX1 (which doesn't yet exist in upstream) were disabled
by the kernel/user.

Note, SGX must be disabled in BIOS to take advantage of this workaround

[*] The additional ENCLS CPUID check on SGX1 exists so that SGX can be
globally "soft" disabled post-reset, e.g. if #MC bits in MCi_CTL are
cleared. Soft disabled meaning disabling SGX without clearing the
primary CPUID bit (in leaf 0x7) and without poking into non-SGX
CPU paths, e.g. for the VMCS controls.

Fixes: 0b665d304028 ("KVM: vmx: Inject #UD for SGX ENCLS instruction in guest")
Reported-by: Toni Spets <toni.spets@iki.fi>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---

This seems somewhat premature given that we don't yet know if the observed
behavior is a logic bug, a one off manufacturing defect, firmware specific,
etc... On the other hand, the change is arguably an optimization
irrespective of using it as a workaround.

arch/x86/kvm/vmx/vmx.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e6138cd5..50cab98382e7 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2338,6 +2338,11 @@ static void hardware_disable(void)
kvm_cpu_vmxoff();
}

+static bool cpu_has_sgx(void)
+{
+ return cpuid_eax(0) >= 0x12 && (cpuid_eax(0x12) & BIT(0));
+}
+
static __init int adjust_vmx_controls(u32 ctl_min, u32 ctl_opt,
u32 msr, u32 *result)
{
@@ -2418,8 +2423,9 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE |
SECONDARY_EXEC_PT_USE_GPA |
SECONDARY_EXEC_PT_CONCEAL_VMX |
- SECONDARY_EXEC_ENABLE_VMFUNC |
- SECONDARY_EXEC_ENCLS_EXITING;
+ SECONDARY_EXEC_ENABLE_VMFUNC;
+ if (cpu_has_sgx())
+ opt2 |= SECONDARY_EXEC_ENCLS_EXITING;
if (adjust_vmx_controls(min2, opt2,
MSR_IA32_VMX_PROCBASED_CTLS2,
&_cpu_based_2nd_exec_control) < 0)
--
2.24.1
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