linux-kernel - Re: [PATCH v3 14/21] KVM:VMX: Add a synthetic MSR to allow userspace to access GUEST

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Message-ID: <791e1cc9-38e1-eb02-673c-13c1194c7960@intel.com>
Date:   Wed, 24 May 2023 10:55:15 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Binbin Wu <binbin.wu@...ux.intel.com>
CC:     <seanjc@...gle.com>, <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <peterz@...radead.org>,
        <rppt@...nel.org>, <rick.p.edgecombe@...el.com>,
        <john.allen@....com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v3 14/21] KVM:VMX: Add a synthetic MSR to allow userspace
 to access GUEST_SSP


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@...el.com>
>> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>>   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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