linux-kernel - Re: [PATCH v7 21/26] KVM: x86: Save and reload SSP to/from SMRAM

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Date:   Fri, 1 Dec 2023 16:55:58 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, <seanjc@...gle.com>,
        <pbonzini@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <dave.hansen@...el.com>, <peterz@...radead.org>,
        <chao.gao@...el.com>, <rick.p.edgecombe@...el.com>,
        <john.allen@....com>
Subject: Re: [PATCH v7 21/26] KVM: x86: Save and reload SSP to/from SMRAM

On 12/1/2023 1:42 AM, Maxim Levitsky wrote:
> On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
>> Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
>> behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
>> at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
>> one of such registers on 64bit Arch, so add the support for SSP.
>>
>> Suggested-by: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>>   arch/x86/kvm/smm.c | 8 ++++++++
>>   arch/x86/kvm/smm.h | 2 +-
>>   2 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
>> index 45c855389ea7..7aac9c54c353 100644
>> --- a/arch/x86/kvm/smm.c
>> +++ b/arch/x86/kvm/smm.c
>> @@ -275,6 +275,10 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
>>   	enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
>>   
>>   	smram->int_shadow = static_call(kvm_x86_get_interrupt_shadow)(vcpu);
>> +
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>> +		KVM_BUG_ON(kvm_msr_read(vcpu, MSR_KVM_SSP, &smram->ssp),
>> +			   vcpu->kvm);
>>   }
>>   #endif
>>   
>> @@ -564,6 +568,10 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
>>   	static_call(kvm_x86_set_interrupt_shadow)(vcpu, 0);
>>   	ctxt->interruptibility = (u8)smstate->int_shadow;
>>   
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>> +		KVM_BUG_ON(kvm_msr_write(vcpu, MSR_KVM_SSP, smstate->ssp),
>> +			   vcpu->kvm);
>> +
>>   	return X86EMUL_CONTINUE;
>>   }
>>   #endif
>> diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
>> index a1cf2ac5bd78..1e2a3e18207f 100644
>> --- a/arch/x86/kvm/smm.h
>> +++ b/arch/x86/kvm/smm.h
>> @@ -116,8 +116,8 @@ struct kvm_smram_state_64 {
>>   	u32 smbase;
>>   	u32 reserved4[5];
>>   
>> -	/* ssp and svm_* fields below are not implemented by KVM */
>>   	u64 ssp;
>> +	/* svm_* fields below are not implemented by KVM */
>>   	u64 svm_guest_pat;
>>   	u64 svm_host_efer;
>>   	u64 svm_host_cr4;
>
> My review feedback from the previous patch series still applies, and I don't
> know why it was not addressed/replied to:
>
> I still think that it is worth it to have a check that CET is not enabled in
> enter_smm_save_state_32 which is called for pure 32 bit guests (guests that don't
> have X86_FEATURE_LM enabled)

I'm not sure whether it's worth doing so for CET on 32-bit guest since you said:
" it lacks several fields because it is no longer maintained ".

I kick the ball to Sean and Paolo to get their voice on this.