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

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 07, 2024, 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.

Heh, if only KVM had a framework that was specifically designed for context
switching MSRs on return to userspace.  Translation: please use the user_return_msr()
APIs.

> Signed-off-by: Xin Li <xin3.li@intel.com>
> Tested-by: Shan Kang <shan.kang@intel.com>
> ---
> 
> 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.

Then grab the host value during vmx_hardware_setup().  And when you rebase on top
of the latest kvm-x86/next, there's a handy dandy "struct kvm_host_values kvm_host"
to track host MSR values (and similar state).

You could also use that for MSR_IA32_FRED_CONFIG and MSR_IA32_FRED_STKLVLS.

> +		 */
> +		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);
> @@ -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);
> +	}
>  #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;

Eh, I wouldn't bother.  Arguably it's better to use __kvm_set_msr(), and "vmx"
is zero-allocated so this is unnecessary.

The GUEST_IA32_FRED_* VMCS fields need to be explicitly initialized because the
VMCS could (very theoretically) use a non-zero-based encoding scheme.