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

[Lai Jiangshan]

On 2021/11/18 19:18, Paolo Bonzini wrote:
> 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.

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 */