linux-kernel - Re: [PATCH 02/15] KVM: VMX: Avoid to rdmsrl(MSR_IA32_SYSENTER

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Message-ID: <94d4b7d8-1e56-69e9-dd52-d154bee6c461@redhat.com>
Date:   Thu, 18 Nov 2021 12:18:39 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 02/15] KVM: VMX: Avoid to rdmsrl(MSR_IA32_SYSENTER_ESP)

On 11/18/21 12:08, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> 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@...ux.alibaba.com>

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 */