Re: [PATCH v9 09/22] KVM: VMX: Save/restore guest FRED RSP0

[Binbin Wu <binbin.wu@linux.intel.com>]

On 10/27/2025 4:18 AM, Xin Li (Intel) wrote:
> From: Xin Li <xin3.li@intel.com>
> 
> Save guest FRED RSP0 in vmx_prepare_switch_to_host() and restore it
> in vmx_prepare_switch_to_guest() because MSR_IA32_FRED_RSP0 is passed
> through to the guest, thus is volatile/unknown.
> 
> Note, host FRED RSP0 is restored in arch_exit_to_user_mode_prepare(),
> regardless of whether it is modified in KVM.
> 
> Signed-off-by: Xin Li <xin3.li@intel.com>
> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> Tested-by: Shan Kang <shan.kang@intel.com>
> Tested-by: Xuelian Guo <xuelian.guo@intel.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
> 
> Changes in v5:
> * Remove the cpu_feature_enabled() check when set/get guest
>   MSR_IA32_FRED_RSP0, as guest_cpu_cap_has() should suffice (Sean).
> * Add a comment when synchronizing current MSR_IA32_FRED_RSP0 MSR to
>   the kernel's local cache, because its handling is different from
>   the MSR_KERNEL_GS_BASE handling (Sean).
> * Add TB from Xuelian Guo.
> 
> Changes in v3:
> * KVM only needs to save/restore guest FRED RSP0 now as host FRED RSP0
>   is restored in arch_exit_to_user_mode_prepare() (Sean Christopherson).
> 
> Changes in v2:
> * Don't use guest_cpuid_has() in vmx_prepare_switch_to_{host,guest}(),
>   which are called from IRQ-disabled context (Chao Gao).
> * Reset msr_guest_fred_rsp0 in __vmx_vcpu_reset() (Chao Gao).
> ---
>  arch/x86/kvm/vmx/vmx.c | 13 +++++++++++++
>  arch/x86/kvm/vmx/vmx.h |  1 +
>  2 files changed, 14 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ef9765779884..c1fb3745247c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1292,6 +1292,9 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
>  	}
>  
>  	wrmsrq(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
> +
> +	if (guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
> +		wrmsrns(MSR_IA32_FRED_RSP0, vmx->msr_guest_fred_rsp0);
>  #else
>  	savesegment(fs, fs_sel);
>  	savesegment(gs, gs_sel);
> @@ -1336,6 +1339,16 @@ static void vmx_prepare_switch_to_host(struct vcpu_vmx *vmx)
>  	invalidate_tss_limit();
>  #ifdef CONFIG_X86_64
>  	wrmsrq(MSR_KERNEL_GS_BASE, vmx->vt.msr_host_kernel_gs_base);
> +
> +	if (guest_cpu_cap_has(&vmx->vcpu, X86_FEATURE_FRED)) {
> +		vmx->msr_guest_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
> +		/*
> +		 * Synchronize the current value in hardware to the kernel's
> +		 * local cache.  The desired host RSP0 will be set when the
> +		 * CPU exits to userspace (RSP0 is a per-task value).
> +		 */
> +		fred_sync_rsp0(vmx->msr_guest_fred_rsp0);
> +	}
>  #endif
>  	load_fixmap_gdt(raw_smp_processor_id());
>  	vmx->vt.guest_state_loaded = false;
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 645b0343e88c..48a5ab12cccf 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -227,6 +227,7 @@ struct vcpu_vmx {
>  	bool                  guest_uret_msrs_loaded;
>  #ifdef CONFIG_X86_64
>  	u64		      msr_guest_kernel_gs_base;
> +	u64		      msr_guest_fred_rsp0;
>  #endif
>  
>  	u64		      spec_ctrl;