Messages in this thread Patch in this message | | | Date | Thu, 18 Nov 2021 22:17:54 +0800 | Subject | Re: [PATCH 02/15] KVM: VMX: Avoid to rdmsrl(MSR_IA32_SYSENTER_ESP) | From | Lai Jiangshan <> |
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On 2021/11/18 19:18, Paolo Bonzini wrote: > On 11/18/21 12:08, Lai Jiangshan wrote: >> From: Lai Jiangshan <laijs@linux.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@linux.alibaba.com> > > Then it's not constant host state, isn't? The hunk in vmx_set_constant_host_state seems wrong. >
The change in vmx_vcpu_load_vmcs() handles only the percpu constant case: (cpu_entry_stack(cpu) + 1), it doesn't handle the case where MSR_IA32_SYSENTER_ESP is NULL.
The change in vmx_set_constant_host_state() handles the case where MSR_IA32_SYSENTER_ESP is NULL, it does be constant host state in this case. If it is not the case, the added code in vmx_vcpu_load_vmcs() will override it safely.
If an else branch with "vmcs_writel(HOST_IA32_SYSENTER_ESP, 0);" is added to vmx_vcpu_load_vmcs(), we will not need to change vmx_set_constant_host_state().
- 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)); + } else { + vmcs_writel(HOST_IA32_SYSENTER_ESP, 0); + }
I'm paranoid about the case when vmcs.HOST_IA32_SYSENTER_ESP is not reset to be NULL when the vcpu is reset. So the hunk in vmx_set_constant_host_state() or the else branch in vmx_vcpu_load_vmcs() is required. I just chose to change the vmx_set_constant_host_state() so that we can reduce a vmcs_writel() in the case where CONFIG_IA32_EMULATION is off in x86_64.
> 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 */ >
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