| Message ID | be14c1e895a2f452047451f050d269217dcee6d9.1653071510.git.maciej.szmigiero@oracle.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | target/i386/kvm: Fix disabling MPX on "-cpu host" with MPX-capable host | expand |
On Fri, May 20, 2022 at 8:33 PM Maciej S. Szmigiero <mail@maciej.szmigiero.name> wrote: > > From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com> > > Since KVM commit 5f76f6f5ff96 ("KVM: nVMX: Do not expose MPX VMX controls when guest MPX disabled") > it is not possible to disable MPX on a "-cpu host" just by adding "-mpx" > there if the host CPU does indeed support MPX. > QEMU will fail to set MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS MSRs in this case > and so trigger an assertion failure. > > Instead, besides "-mpx" one has to explicitly add also > "-vmx-exit-clear-bndcfgs" and "-vmx-entry-load-bndcfgs" to QEMU command > line to make it work, which is a bit convoluted. > > Sanitize MPX-related bits in MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS after > setting the vCPU CPUID instead so such workarounds are no longer necessary. Can you use feature_dependencies instead? See for example { .from = { FEAT_7_0_EBX, CPUID_7_0_EBX_RDSEED }, .to = { FEAT_VMX_SECONDARY_CTLS, VMX_SECONDARY_EXEC_RDSEED_EXITING }, }, Paolo
On 21.05.2022 10:54, Paolo Bonzini wrote: > On Fri, May 20, 2022 at 8:33 PM Maciej S. Szmigiero > <mail@maciej.szmigiero.name> wrote: >> >> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com> >> >> Since KVM commit 5f76f6f5ff96 ("KVM: nVMX: Do not expose MPX VMX controls when guest MPX disabled") >> it is not possible to disable MPX on a "-cpu host" just by adding "-mpx" >> there if the host CPU does indeed support MPX. >> QEMU will fail to set MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS MSRs in this case >> and so trigger an assertion failure. >> >> Instead, besides "-mpx" one has to explicitly add also >> "-vmx-exit-clear-bndcfgs" and "-vmx-entry-load-bndcfgs" to QEMU command >> line to make it work, which is a bit convoluted. >> >> Sanitize MPX-related bits in MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS after >> setting the vCPU CPUID instead so such workarounds are no longer necessary. > > Can you use feature_dependencies instead? See for example > > { > .from = { FEAT_7_0_EBX, CPUID_7_0_EBX_RDSEED }, > .to = { FEAT_VMX_SECONDARY_CTLS, VMX_SECONDARY_EXEC_RDSEED_EXITING }, > }, The "feature_dependencies" way seems to work fine too, and it definitely looks neater. Thanks for the pointer, will post v2 in a moment. > Paolo > Thanks, Maciej
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c index a9ee8eebd7..435cb18753 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -2934,6 +2934,17 @@ static uint64_t make_vmx_msr_value(uint32_t index, uint32_t features) return must_be_one | (((uint64_t)can_be_one) << 32); } +static void kvm_msr_entry_add_if_supported(X86CPU *cpu, uint32_t msr, + uint32_t mask, uint32_t features, + uint64_t value_or) +{ + uint32_t supported = + kvm_arch_get_supported_msr_feature(kvm_state, msr) >> 32; + uint32_t feat_eff = features & (~mask | (mask & supported)); + + kvm_msr_entry_add(cpu, msr, make_vmx_msr_value(msr, feat_eff) | value_or); +} + static void kvm_msr_entry_add_vmx(X86CPU *cpu, FeatureWordArray f) { uint64_t kvm_vmx_basic = @@ -2996,12 +3007,23 @@ static void kvm_msr_entry_add_vmx(X86CPU *cpu, FeatureWordArray f) kvm_msr_entry_add(cpu, MSR_IA32_VMX_TRUE_PINBASED_CTLS, make_vmx_msr_value(MSR_IA32_VMX_TRUE_PINBASED_CTLS, f[FEAT_VMX_PINBASED_CTLS])); - kvm_msr_entry_add(cpu, MSR_IA32_VMX_TRUE_EXIT_CTLS, - make_vmx_msr_value(MSR_IA32_VMX_TRUE_EXIT_CTLS, - f[FEAT_VMX_EXIT_CTLS]) | fixed_vmx_exit); - kvm_msr_entry_add(cpu, MSR_IA32_VMX_TRUE_ENTRY_CTLS, - make_vmx_msr_value(MSR_IA32_VMX_TRUE_ENTRY_CTLS, - f[FEAT_VMX_ENTRY_CTLS])); + + /* + * When disabling MPX on a host that supports this function it is not + * enough to clear the relevant CPUID bit, MPX-related bits in + * MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS have to be cleared, too. + * + * Otherwise setting these MSRs will fail. + */ + kvm_msr_entry_add_if_supported(cpu, MSR_IA32_VMX_TRUE_EXIT_CTLS, + VMX_VM_EXIT_CLEAR_BNDCFGS, + f[FEAT_VMX_EXIT_CTLS], + fixed_vmx_exit); + kvm_msr_entry_add_if_supported(cpu, MSR_IA32_VMX_TRUE_ENTRY_CTLS, + VMX_VM_ENTRY_LOAD_BNDCFGS, + f[FEAT_VMX_ENTRY_CTLS], + 0); + kvm_msr_entry_add(cpu, MSR_IA32_VMX_PROCBASED_CTLS2, make_vmx_msr_value(MSR_IA32_VMX_PROCBASED_CTLS2, f[FEAT_VMX_SECONDARY_CTLS]));