Re: [PATCH v2 1/3] KVM: x86: Allow CPU to force vendor-specific TDP level

[Yu Zhang]

On Sun, Aug 08, 2021 at 11:11:40PM -0500, Wei Huang wrote:
> 
> 
> On 8/8/21 10:58 PM, Yu Zhang wrote:
> > On Sun, Aug 08, 2021 at 02:26:56PM -0500, Wei Huang wrote:
> > > AMD future CPUs will require a 5-level NPT if host CR4.LA57 is set.
> > 
> > Sorry, but why? NPT is not indexed by HVA.
> 
> NPT is not indexed by HVA - it is always indexed by GPA. What I meant is NPT
> page table level has to be the same as the host OS page table: if 5-level
> page table is enabled in host OS (CR4.LA57=1), guest NPT has to 5-level too.

I know what you meant. But may I ask why? 

B.R.
Yu

> 
> > 
> > > To prevent kvm_mmu_get_tdp_level() from incorrectly changing NPT level
> > > on behalf of CPUs, add a new parameter in kvm_configure_mmu() to force
> > > a fixed TDP level.
> > > 
> > > Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
> > > ---
> > >   arch/x86/include/asm/kvm_host.h |  5 ++---
> > >   arch/x86/kvm/mmu/mmu.c          | 10 ++++++++--
> > >   arch/x86/kvm/svm/svm.c          |  4 +++-
> > >   arch/x86/kvm/vmx/vmx.c          |  3 ++-
> > >   4 files changed, 15 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > index 974cbfb1eefe..6d16f75cc8da 100644
> > > --- a/arch/x86/include/asm/kvm_host.h
> > > +++ b/arch/x86/include/asm/kvm_host.h
> > > @@ -723,7 +723,6 @@ struct kvm_vcpu_arch {
> > >   	u64 reserved_gpa_bits;
> > >   	int maxphyaddr;
> > > -	int max_tdp_level;
> > >   	/* emulate context */
> > > @@ -1747,8 +1746,8 @@ void kvm_mmu_invalidate_gva(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> > >   void kvm_mmu_invpcid_gva(struct kvm_vcpu *vcpu, gva_t gva, unsigned long pcid);
> > >   void kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd);
> > > -void kvm_configure_mmu(bool enable_tdp, int tdp_max_root_level,
> > > -		       int tdp_huge_page_level);
> > > +void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
> > > +		       int tdp_max_root_level, int tdp_huge_page_level);
> > >   static inline u16 kvm_read_ldt(void)
> > >   {
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 66f7f5bc3482..c11ee4531f6d 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -97,6 +97,7 @@ module_param_named(flush_on_reuse, force_flush_and_sync_on_reuse, bool, 0644);
> > >   bool tdp_enabled = false;
> > >   static int max_huge_page_level __read_mostly;
> > > +static int tdp_root_level __read_mostly;
> > 
> > I think this is a broken design - meaning KVM can only use 5-level or
> > 4-level NPT for all VMs.
> 
> Broken normally means non-functional or buggy, which doesn't apply here. A
> good TLB design should be able to offset the potential overhead of 5-level
> page table for most cases.
> 
> > 
> > B.R.
> > Yu
> > 
> > >   static int max_tdp_level __read_mostly;
> > >   enum {
> > > @@ -4562,6 +4563,10 @@ static union kvm_mmu_role kvm_calc_mmu_role_common(struct kvm_vcpu *vcpu,
> > >   static inline int kvm_mmu_get_tdp_level(struct kvm_vcpu *vcpu)
> > >   {
> > > +	/* tdp_root_level is architecture forced level, use it if nonzero */
> > > +	if (tdp_root_level)
> > > +		return tdp_root_level;
> > > +
> > >   	/* Use 5-level TDP if and only if it's useful/necessary. */
> > >   	if (max_tdp_level == 5 && cpuid_maxphyaddr(vcpu) <= 48)
> > >   		return 4;
> > > @@ -5253,10 +5258,11 @@ void kvm_mmu_invpcid_gva(struct kvm_vcpu *vcpu, gva_t gva, unsigned long pcid)
> > >   	 */
> > >   }
> > > -void kvm_configure_mmu(bool enable_tdp, int tdp_max_root_level,
> > > -		       int tdp_huge_page_level)
> > > +void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
> > > +		       int tdp_max_root_level, int tdp_huge_page_level)
> > >   {
> > >   	tdp_enabled = enable_tdp;
> > > +	tdp_root_level = tdp_forced_root_level;
> > >   	max_tdp_level = tdp_max_root_level;
> > >   	/*
> > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > index e8ccab50ebf6..f361d466e18e 100644
> > > --- a/arch/x86/kvm/svm/svm.c
> > > +++ b/arch/x86/kvm/svm/svm.c
> > > @@ -1015,7 +1015,9 @@ static __init int svm_hardware_setup(void)
> > >   	if (!boot_cpu_has(X86_FEATURE_NPT))
> > >   		npt_enabled = false;
> > > -	kvm_configure_mmu(npt_enabled, get_max_npt_level(), PG_LEVEL_1G);
> > > +	/* Force VM NPT level equal to the host's max NPT level */
> > > +	kvm_configure_mmu(npt_enabled, get_max_npt_level(),
> > > +			  get_max_npt_level(), PG_LEVEL_1G);
> > >   	pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
> > >   	/* Note, SEV setup consumes npt_enabled. */
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 927a552393b9..034e1397c7d5 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -7803,7 +7803,8 @@ static __init int hardware_setup(void)
> > >   		ept_lpage_level = PG_LEVEL_2M;
> > >   	else
> > >   		ept_lpage_level = PG_LEVEL_4K;
> > > -	kvm_configure_mmu(enable_ept, vmx_get_max_tdp_level(), ept_lpage_level);
> > > +	kvm_configure_mmu(enable_ept, 0, vmx_get_max_tdp_level(),
> > > +			  ept_lpage_level);
> > >   	/*
> > >   	 * Only enable PML when hardware supports PML feature, and both EPT
> > > -- 
> > > 2.31.1
> > >