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

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Date:   Mon, 16 Oct 2017 12:53:56 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Wanpeng Li <kernellwp@...il.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

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.

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
>