Re: [PATCH v2 09/25] KVM: VMX: Switch FRED RSP0 between host and guest

[Chao Gao]

On Wed, Feb 07, 2024 at 09:26:29AM -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.
>
>Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
>Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
>---
>
>Changes since v1:
>* 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 | 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 b7b772183ee4..264378c3b784 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -1337,6 +1337,16 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
> 	}
> 
> 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
>+
>+	if (guest_can_use(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);

can we just drop this and use "(unsigned long)task_stack_page(current) + THREAD_SIZE"
as host fred rsp0?

>+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_guest_fred_rsp0);

any reason to not use wrmsrns?

>+	}
> #else
> 	savesegment(fs, fs_sel);
> 	savesegment(gs, gs_sel);
>@@ -1381,6 +1391,11 @@ 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 (guest_can_use(&vmx->vcpu, X86_FEATURE_FRED)) {
>+		vmx->msr_guest_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
>+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_host_fred_rsp0);

same question.

>+	}
> #endif
> 	load_fixmap_gdt(raw_smp_processor_id());
> 	vmx->guest_state_loaded = false;
>@@ -4889,6 +4904,8 @@ static void __vmx_vcpu_reset(struct kvm_vcpu *vcpu)
> 
> #ifdef CONFIG_X86_64
> 	if (kvm_cpu_cap_has(X86_FEATURE_FRED)) {
>+		vmx->msr_guest_fred_rsp0 = 0;
>+
> 		vmcs_write64(GUEST_IA32_FRED_CONFIG, 0);
> 		vmcs_write64(GUEST_IA32_FRED_RSP1, 0);
> 		vmcs_write64(GUEST_IA32_FRED_RSP2, 0);
>diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
>index 3ad52437f426..176ad39be406 100644
>--- a/arch/x86/kvm/vmx/vmx.h
>+++ b/arch/x86/kvm/vmx/vmx.h
>@@ -278,6 +278,8 @@ 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;
> #endif
> 
> 	u64		      spec_ctrl;
>-- 
>2.43.0
>
>