linux-kernel - Re: [PATCH 2/2] KVM: VMX: Fix VPID capability detection

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Date:   Tue, 17 Oct 2017 10:20:09 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 2/2] KVM: VMX: Fix VPID capability detection

Hi Jim,
2017-10-17 3:53 GMT+08:00 Jim Mattson <jmattson@...gle.com>:
> According to the SDM,
>
> The IA32_VMX_EPT_VPID_CAP MSR exists only on processors that support
> the 1-setting of the “activate secondary
> controls” VM-execution control (only if bit 63 of the
> IA32_VMX_PROCBASED_CTLS MSR is 1) and that support
> either the 1-setting of the “enable EPT” VM-execution control (only if
> bit 33 of the IA32_VMX_PROCBASED_CTLS2
> MSR is 1) or the 1-setting of the “enable VPID” VM-execution control
> (only if bit 37 of the
> IA32_VMX_PROCBASED_CTLS2 MSR is 1).
>
> Therefore, it seems dangerous to hoist the
> rdmsr(MSR_IA32_VMX_EPT_VPID_CAP, ...) outside of the conditional,
> unless you change it to rdmsr_safe.
>
> I don't think Intel has ever shipped a CPU with VPID but without EPT,
> so the motivation for this change is unclear to me.

My fault, I need to add more explanation to both patch 1/2, 2/2,
actually both patches are try to handle issues observed on L1 when EPT
is not exposed to L1.

For patch 1/2:
EPT switching is emulated for nested, however, I still can use vmxcap
tool to observe "EPTP Switching   yes" even if EPT is not exposed to
L1.
For patch 2/2:
EPT is not exposed to L1, the VPID capability is exposed and can be
observed by vmxcap tool:
  INVVPID supported                        yes
  Individual-address INVVPID               yes
  Single-context INVVPID                   yes
  All-context INVVPID                      yes
  Single-context-retaining-globals INVVPID yes

However, the module parameter of VPID observed in L1 is always N, the
cpu_has_vmx_invvpid() check in L1 KVM fails since vmx_capability.vpid
is 0 and it is not read from MSR due to EPT is not exposed. This is
what patch 2/2 tries to fix.

Regards,
Wanpeng Li

>
> On Sat, Oct 14, 2017 at 7:31 PM, Wanpeng Li <kernellwp@...il.com> wrote:
>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>> According to the Intel SDM, volume 3, section 28.3.2: Creating and
>> Using Cached Translation Information, "The following items describe the
>> creation of mappings while EPT is not in use":
>>  - Linear mappings may be created. They are derived from the paging
>>    structures referenced (directly or indirectly) by the current value
>>    of CR3 and are associated with the current VPID and the current PCID.
>>
>> The VPID is used to tag linear mappings when EPT is not enabled. However,
>> current logic just detects VPID capability if EPT is enabled, this patch
>> fixes it.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>>  arch/x86/kvm/vmx.c | 10 +++++++---
>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 3644540..800d08c 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -3681,15 +3681,19 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
>>                                 SECONDARY_EXEC_VIRTUALIZE_X2APIC_MODE |
>>                                 SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY);
>>
>> +       rdmsr(MSR_IA32_VMX_EPT_VPID_CAP,
>> +               vmx_capability.ept, vmx_capability.vpid);
>> +
>>         if (_cpu_based_2nd_exec_control & SECONDARY_EXEC_ENABLE_EPT) {
>>                 /* CR3 accesses and invlpg don't need to cause VM Exits when EPT
>>                    enabled */
>>                 _cpu_based_exec_control &= ~(CPU_BASED_CR3_LOAD_EXITING |
>>                                              CPU_BASED_CR3_STORE_EXITING |
>>                                              CPU_BASED_INVLPG_EXITING);
>> -               rdmsr(MSR_IA32_VMX_EPT_VPID_CAP,
>> -                     vmx_capability.ept, vmx_capability.vpid);
>> -       }
>> +       } else
>> +               vmx_capability.ept = 0;
>> +       if (!(_cpu_based_2nd_exec_control & SECONDARY_EXEC_ENABLE_VPID))
>> +               vmx_capability.vpid = 0;
>>
>>         min = VM_EXIT_SAVE_DEBUG_CONTROLS | VM_EXIT_ACK_INTR_ON_EXIT;
>>  #ifdef CONFIG_X86_64
>> --
>> 2.7.4
>>