linux-kernel - Re: [PATCH v8 18/21] KVM: nVMX: Add FRED VMCS fields to nested VMX context handling

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Message-ID: <7f33ee53-1569-4a47-8018-fd48e9c65901@zytor.com>
Date: Tue, 14 Oct 2025 15:49:59 +0800
From: Xin Li <xin@...or.com>
To: Chao Gao <chao.gao@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, pbonzini@...hat.com, seanjc@...gle.com,
        corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, peterz@...radead.org, andrew.cooper3@...rix.com,
        hch@...radead.org
Subject: Re: [PATCH v8 18/21] KVM: nVMX: Add FRED VMCS fields to nested VMX
 context handling

On 10/14/2025 2:57 PM, Chao Gao wrote:
>> +	if (!vmx->nested.nested_run_pending ||
>> +	    !nested_cpu_load_guest_fred_state(vmcs12)) {
>> +		vmx->nested.pre_vmenter_fred_config = vmcs_read64(GUEST_IA32_FRED_CONFIG);
>> +		vmx->nested.pre_vmenter_fred_rsp1 = vmcs_read64(GUEST_IA32_FRED_RSP1);
>> +		vmx->nested.pre_vmenter_fred_rsp2 = vmcs_read64(GUEST_IA32_FRED_RSP2);
>> +		vmx->nested.pre_vmenter_fred_rsp3 = vmcs_read64(GUEST_IA32_FRED_RSP3);
>> +		vmx->nested.pre_vmenter_fred_stklvls = vmcs_read64(GUEST_IA32_FRED_STKLVLS);
>> +		vmx->nested.pre_vmenter_fred_ssp1 = vmcs_read64(GUEST_IA32_FRED_SSP1);
>> +		vmx->nested.pre_vmenter_fred_ssp2 = vmcs_read64(GUEST_IA32_FRED_SSP2);
>> +		vmx->nested.pre_vmenter_fred_ssp3 = vmcs_read64(GUEST_IA32_FRED_SSP3);
> FRED state save/restore is needed only when the guest has FRED support. So, the
> above save/restore should be guarded by guest_cpu_cap_has(vcpu, X86_FEATURE_FRED).
> Otherwise, on hardware without FRED, vmread_error() would be triggered here as
> reported by syzbot.

You're right, I wanted to avoid unnecessary checking, but this caught me.

Will fix it with a v8A patch.