Re: [PATCH v4] KVM: nVMX: Fully support of nested VMX preemption timer

[Gleb Natapov <gleb@redhat.com>]

On Fri, Sep 13, 2013 at 07:15:11PM +0200, Paolo Bonzini wrote:
> Il 06/09/2013 04:04, Arthur Chunqi Li ha scritto:
> > This patch contains the following two changes:
> > 1. Fix the bug in nested preemption timer support. If vmexit L2->L0
> > with some reasons not emulated by L1, preemption timer value should
> > be save in such exits.
> > 2. Add support of "Save VMX-preemption timer value" VM-Exit controls
> > to nVMX.
> > 
> > With this patch, nested VMX preemption timer features are fully
> > supported.
> > 
> > Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>
> > ---
> > ChangeLog to v3:
> > Move nested_adjust_preemption_timer to the latest place just before vmenter.
> > Some minor changes.
> > 
> >  arch/x86/include/uapi/asm/msr-index.h |    1 +
> >  arch/x86/kvm/vmx.c                    |   49 +++++++++++++++++++++++++++++++--
> >  2 files changed, 48 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/include/uapi/asm/msr-index.h b/arch/x86/include/uapi/asm/msr-index.h
> > index bb04650..b93e09a 100644
> > --- a/arch/x86/include/uapi/asm/msr-index.h
> > +++ b/arch/x86/include/uapi/asm/msr-index.h
> > @@ -536,6 +536,7 @@
> >  
> >  /* MSR_IA32_VMX_MISC bits */
> >  #define MSR_IA32_VMX_MISC_VMWRITE_SHADOW_RO_FIELDS (1ULL << 29)
> > +#define MSR_IA32_VMX_MISC_PREEMPTION_TIMER_SCALE   0x1F
> >  /* AMD-V MSRs */
> >  
> >  #define MSR_VM_CR                       0xc0010114
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index 1f1da43..f364d16 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -374,6 +374,8 @@ struct nested_vmx {
> >  	 */
> >  	struct page *apic_access_page;
> >  	u64 msr_ia32_feature_control;
> > +	/* Set if vmexit is L2->L1 */
> > +	bool nested_vmx_exit;
> >  };
> >  
> >  #define POSTED_INTR_ON  0
> > @@ -2204,7 +2206,17 @@ static __init void nested_vmx_setup_ctls_msrs(void)
> >  #ifdef CONFIG_X86_64
> >  		VM_EXIT_HOST_ADDR_SPACE_SIZE |
> >  #endif
> > -		VM_EXIT_LOAD_IA32_PAT | VM_EXIT_SAVE_IA32_PAT;
> > +		VM_EXIT_LOAD_IA32_PAT | VM_EXIT_SAVE_IA32_PAT |
> > +		VM_EXIT_SAVE_VMX_PREEMPTION_TIMER;
> > +	if (!(nested_vmx_pinbased_ctls_high &
> > +				PIN_BASED_VMX_PREEMPTION_TIMER) ||
> > +			!(nested_vmx_exit_ctls_high &
> > +				VM_EXIT_SAVE_VMX_PREEMPTION_TIMER)) {
> 
> Align this under the other "!".  Also, I prefer to have one long line
> for the whole "!(... & ...) ||" (and likewise below), but I don't know
> if Gleb agrees
> 
!(... & ...) ||
!(... & ...)

fits perfectly to 80 chars.

> > +		nested_vmx_exit_ctls_high &=
> > +			(~VM_EXIT_SAVE_VMX_PREEMPTION_TIMER);
> 
> Please remove parentheses around ~VM_EXIT_SAVE_VMX_PREEMPTION_TIMER, and
> likewise elsewhere in the patch.
> 
and line break too.

> > +		nested_vmx_pinbased_ctls_high &=
> > +			(~PIN_BASED_VMX_PREEMPTION_TIMER);
> > +	}
> >  	nested_vmx_exit_ctls_high |= (VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR |
> >  				      VM_EXIT_LOAD_IA32_EFER);
> >  
> > @@ -6707,6 +6719,24 @@ static void vmx_get_exit_info(struct kvm_vcpu *vcpu, u64 *info1, u64 *info2)
> >  	*info2 = vmcs_read32(VM_EXIT_INTR_INFO);
> >  }
> >  
> > +static void nested_adjust_preemption_timer(struct kvm_vcpu *vcpu)
> > +{
> > +	u64 delta_tsc_l1;
> > +	u32 preempt_val_l1, preempt_val_l2, preempt_scale;
> 
> Should this exit immediately if the preemption timer pin-based control
> is disabled?
> 
> > +	preempt_scale = native_read_msr(MSR_IA32_VMX_MISC) &
> > +			MSR_IA32_VMX_MISC_PREEMPTION_TIMER_SCALE;
> > +	preempt_val_l2 = vmcs_read32(VMX_PREEMPTION_TIMER_VALUE);
> > +	delta_tsc_l1 = kvm_x86_ops->read_l1_tsc(vcpu,
> > +			native_read_tsc()) - vcpu->arch.last_guest_tsc;
> 
> Please format this like:
> 
> 	delta_tsc_l1 =
> 		kvm_x86_ops->read_l1_tsc(vcpu, native_read_tsc())
> 		- vcpu->arch.last_guest_tsc;
> 
And call vmx_read_l1_tsc() directly. Actually you can even use
to_vmx(vcpu)->nested.vmcs01_tsc_offset directly here since the function
will be called only when is_guest_mode() == true, but vmx_read_l1_tsc()
may be more clear here and compile should optimize second is_guest_mode()
check anyway.

> > +	preempt_val_l1 = delta_tsc_l1 >> preempt_scale;
> > +	if (preempt_val_l2 <= preempt_val_l1)
> > +		preempt_val_l2 = 0;
> > +	else
> > +		preempt_val_l2 -= preempt_val_l1;
> > +	vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, preempt_val_l2);
> 
> Did you test that a value of 0 triggers an immediate exit, rather than
> counting down by 2^32?  Perhaps it's safer to limit the value to 1
> instead of 0.
> 
> > +}
> > +
> >  /*
> >   * The guest has exited.  See if we can fix it or if we need userspace
> >   * assistance.
> > @@ -6736,9 +6766,11 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
> >  		vmx->nested.nested_run_pending = 0;
> >  
> >  	if (is_guest_mode(vcpu) && nested_vmx_exit_handled(vcpu)) {
> > +		vmx->nested.nested_vmx_exit = true;
> 
> I think this assignment should be in nested_vmx_vmexit, since it is
> called from other places as well.
> 
> >  		nested_vmx_vmexit(vcpu);
> >  		return 1;
> >  	}
> > +	vmx->nested.nested_vmx_exit = false;
> >  
> >  	if (exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) {
> >  		vcpu->run->exit_reason = KVM_EXIT_FAIL_ENTRY;
> > @@ -7132,6 +7164,8 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >  	debugctlmsr = get_debugctlmsr();
> >  
> >  	vmx->__launched = vmx->loaded_vmcs->launched;
> > +	if (is_guest_mode(vcpu) && !(vmx->nested.nested_vmx_exit))
> > +		nested_adjust_preemption_timer(vcpu);
> 
> Please leave the assignment to __launched last, since it's already
> initializing the asm below.
> 
> I don't like the is_guest_mode check here... Maybe it's
> micro-optimizing, but I wonder if we already do too many checks in
> vmx_vcpu_run...  For example, is_guest_mode could be changed (I think)
> to a check for "vmx->loaded_vmcs == &vmx->vmcs1".
> 
Why this will be more efficient that HF_GUEST_MASK check?

> Alternatively, we could change nested_vmx_exit to an enum in struct
> vcpu_vmx (with values for L0->L1, L0->L2, L1->L2) that is initialized in
> vmx_handle_exit.  Then we could check directly for L0->L2 and not adjust
> the preemption timer in other cases.  In fact, I suspect this enum could
> replace HF_GUEST_MASK altogether.  However, this would require some
> other, more complicated, changes to svm.c.
> 
> Gleb, what do you think?
> 
I do not see why nested_vmx_exit is necessary at all yet. We can detect
all aforementioned cases without.

--
			Gleb.