Re: [PATCH 02/15] KVM: VMX: Avoid to rdmsrl(MSR_IA32_SYSENTER_ESP)

[Paolo Bonzini]

On 11/18/21 12:08, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
>
> The value of host MSR_IA32_SYSENTER_ESP is known to be constant for
> each CPU: (cpu_entry_stack(cpu) + 1) when 32 bit syscall is enabled or
> NULL is 32 bit syscall is not enabled.
>
> So rdmsrl() can be avoided for the first case and both rdmsrl() and
> vmcs_writel() can be avoided for the second case.
>
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

Then it's not constant host state, isn't?  The hunk in 
vmx_set_constant_host_state seems wrong.

Paolo

> ---
>   arch/x86/kvm/vmx/vmx.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 48a34d1a2989..4e4a33226edb 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1271,7 +1271,6 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
>   
>   	if (!already_loaded) {
>   		void *gdt = get_current_gdt_ro();
> -		unsigned long sysenter_esp;
>   
>   		/*
>   		 * Flush all EPTP/VPID contexts, the new pCPU may have stale
> @@ -1287,8 +1286,11 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
>   			    (unsigned long)&get_cpu_entry_area(cpu)->tss.x86_tss);
>   		vmcs_writel(HOST_GDTR_BASE, (unsigned long)gdt);   /* 22.2.4 */
>   
> -		rdmsrl(MSR_IA32_SYSENTER_ESP, sysenter_esp);
> -		vmcs_writel(HOST_IA32_SYSENTER_ESP, sysenter_esp); /* 22.2.3 */
> +		if (IS_ENABLED(CONFIG_IA32_EMULATION) || IS_ENABLED(CONFIG_X86_32)) {
> +			/* 22.2.3 */
> +			vmcs_writel(HOST_IA32_SYSENTER_ESP,
> +				    (unsigned long)(cpu_entry_stack(cpu) + 1));
> +		}
>   
>   		vmx->loaded_vmcs->cpu = cpu;
>   	}
> @@ -4021,6 +4023,8 @@ void vmx_set_constant_host_state(struct vcpu_vmx *vmx)
>   
>   	rdmsr(MSR_IA32_SYSENTER_CS, low32, high32);
>   	vmcs_write32(HOST_IA32_SYSENTER_CS, low32);
> +	rdmsrl(MSR_IA32_SYSENTER_ESP, tmpl);
> +	vmcs_writel(HOST_IA32_SYSENTER_ESP, tmpl);
>   	rdmsrl(MSR_IA32_SYSENTER_EIP, tmpl);
>   	vmcs_writel(HOST_IA32_SYSENTER_EIP, tmpl);   /* 22.2.3 */