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

Yu Zhang
2021-08-09T03:58:21Z

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.

> 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@amd.com>
> ---
> 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.

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
>