Re: [PATCH v1 09/23] KVM: VMX: Switch FRED RSP0 between host and guest

[Chao Gao <chao.gao@intel.com>]

On Wed, Nov 08, 2023 at 10:29:49AM -0800, Xin Li wrote:
>Switch MSR_IA32_FRED_RSP0 between host and guest in
>vmx_prepare_switch_to_{host,guest}().
>
>MSR_IA32_FRED_RSP0 is used during ring 3 event delivery only, thus
>KVM, running on ring 0, can run safely with guest FRED RSP0, i.e.,
>no need to switch between host/guest FRED RSP0 during VM entry and
>exit.
>
>KVM should switch to host FRED RSP0 before returning to user level,
>and switch to guest FRED RSP0 before entering guest mode.
>
>Tested-by: Shan Kang <shan.kang@intel.com>
>Signed-off-by: Xin Li <xin3.li@intel.com>
>---
> arch/x86/kvm/vmx/vmx.c | 17 +++++++++++++++++
> arch/x86/kvm/vmx/vmx.h |  2 ++
> 2 files changed, 19 insertions(+)
>
>diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>index 41772ecdd368..d00ab9d4c93e 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -1344,6 +1344,17 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
> 	}
> 
> 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
>+
>+	if (cpu_feature_enabled(X86_FEATURE_FRED) &&
>+	    guest_cpuid_has(vcpu, X86_FEATURE_FRED)) {
>+		/*
>+		 * MSR_IA32_FRED_RSP0 is top of task stack, which never changes.
>+		 * Thus it should be initialized only once.
>+		 */
>+		if (unlikely(vmx->msr_host_fred_rsp0 == 0))
>+			vmx->msr_host_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
>+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_guest_fred_rsp0);
>+	}
> #else
> 	savesegment(fs, fs_sel);
> 	savesegment(gs, gs_sel);
>@@ -1388,6 +1399,12 @@ static void vmx_prepare_switch_to_host(struct vcpu_vmx *vmx)
> 	invalidate_tss_limit();
> #ifdef CONFIG_X86_64
> 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
>+
>+	if (cpu_feature_enabled(X86_FEATURE_FRED) &&
>+	    guest_cpuid_has(&vmx->vcpu, X86_FEATURE_FRED)) {

IIUC, vmx_prepare_switch_to_host() is called from IRQ-disabled context. using
guest_cpuid_has() in this context is not desired, see lockdep_assert_irqs_enabled()
in cpuid_entry2_find().

>+		vmx->msr_guest_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
>+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_host_fred_rsp0);
>+	}
> #endif
> 	load_fixmap_gdt(raw_smp_processor_id());
> 	vmx->guest_state_loaded = false;
>diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
>index f8c02bd37069..328a3447f064 100644
>--- a/arch/x86/kvm/vmx/vmx.h
>+++ b/arch/x86/kvm/vmx/vmx.h
>@@ -276,6 +276,8 @@ struct vcpu_vmx {
> #ifdef CONFIG_X86_64
> 	u64		      msr_host_kernel_gs_base;
> 	u64		      msr_guest_kernel_gs_base;
>+	u64		      msr_host_fred_rsp0;
>+	u64		      msr_guest_fred_rsp0;

resetting guest fred rsp0 to 0 during vcpu reset is missing.

> #endif
> 
> 	u64		      spec_ctrl;
>-- 
>2.42.0
>
>