Re: [PATCH v3 14/21] KVM:VMX: Add a synthetic MSR to allow userspace to access GUEST_SSP

[Yang, Weijiang]

On 5/23/2023 4:57 PM, Binbin Wu wrote:
> On 5/11/2023 12:08 PM, Yang Weijiang wrote:
> > Introduce a host-only synthetic MSR, MSR_KVM_GUEST_SSP, so that the VMM
> > can read/write the guest's SSP, e.g. to migrate CET state.  Use a 
> > synthetic
> > MSR, e.g. as opposed to a VCPU_REG_, as GUEST_SSP is subject to the same
> > consistency checks as the PL*_SSP MSRs, i.e. can share code.
> >
> > Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >   arch/x86/include/uapi/asm/kvm_para.h |  1 +
> >   arch/x86/kvm/vmx/vmx.c               | 15 ++++++++++++---
> >   arch/x86/kvm/x86.c                   |  4 ++++
> >   3 files changed, 17 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/include/uapi/asm/kvm_para.h 
> > b/arch/x86/include/uapi/asm/kvm_para.h
> > index 6e64b27b2c1e..7af465e4e0bd 100644
> > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > @@ -58,6 +58,7 @@
> >   #define MSR_KVM_ASYNC_PF_INT    0x4b564d06
> >   #define MSR_KVM_ASYNC_PF_ACK    0x4b564d07
> >   #define MSR_KVM_MIGRATION_CONTROL    0x4b564d08
> > +#define MSR_KVM_GUEST_SSP    0x4b564d09
> >     struct kvm_steal_time {
> >       __u64 steal;
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 0ccaa467d7d3..72149156bbd3 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -2095,9 +2095,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, 
> > struct msr_data *msr_info)
> >           break;
> >       case MSR_IA32_U_CET:
> >       case MSR_IA32_PL3_SSP:
> > +    case MSR_KVM_GUEST_SSP:
> >           if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
> >               return 1;
> > -        kvm_get_xsave_msr(msr_info);
> > +        if (msr_info->index == MSR_KVM_GUEST_SSP)
> > +            msr_info->data = vmcs_readl(GUEST_SSP);
> According to the change of the kvm_cet_is_msr_accessible() below,
> kvm_cet_is_msr_accessible() will return false for MSR_KVM_GUEST_SSP, 
> then this code is unreachable?

No, when the access is initiated from host side, 
kvm_cet_is_msr_accessible() return true for MSR_KVM_GUEST_SSP.

So the code is reachable:

if (msr->host_initiated)

    return true;


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