Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

[Jeremi Piotrowski]

On 06/03/2023 18:52, Vitaly Kuznetsov wrote:
> Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx> writes:
> 
>> TDP MMU has been broken on AMD CPUs when running on Hyper-V since v5.17.
>> The issue was first introduced by two commmits:
>>
>> - bb95dfb9e2dfbe6b3f5eb5e8a20e0259dadbe906 "KVM: x86/mmu: Defer TLB
>>   flush to caller when freeing TDP MMU shadow pages"
>> - efd995dae5eba57c5d28d6886a85298b390a4f07 "KVM: x86/mmu: Zap defunct
>>   roots via asynchronous worker"
>>
>> The root cause is that since then there are missing TLB flushes which
>> are required by HV_X64_NESTED_ENLIGHTENED_TLB.
> 
> Please share more details on what's actually missing as you get them,
> I'd like to understand which flushes can be legally avoided on bare
> hardware and Hyper-V/VMX but not on Hyper-V/SVM.
> 

See the linked thread here
https://lore.kernel.org/lkml/20d189fc-8d20-8083-b448-460cc0420151@xxxxxxxxxxxxxxxxxxx/#t
for all the details/analyses but the summary was that either of these 2
options would work, with a) having less flushes (footnote: less flushes is not necessarily
better):

a) adding a hyperv_flush_guest_mapping(__pa(root->spt) after kvm_tdp_mmu_get_vcpu_root_hpa's call to tdp_mmu_alloc_sp()
b) adding a hyperv_flush_guest_mapping(vcpu->arch.mmu->root.hpa) to svm_flush_tlb_current()

These are only needed on Hyper-V/SVM because of how the enlightenment works (needs an explicit
flush to rebuild L0 shadow page tables). Hyper-V/VMX does not need any changes and currently
works. Let me know if you need more information on something here, I'll try to get it.

>>  The failure manifests
>> as L2 guest VMs being unable to complete boot due to memory
>> inconsistencies between L1 and L2 guests which lead to various
>> assertion/emulation failures.
>>
>> The HV_X64_NESTED_ENLIGHTENED_TLB enlightenment is always exposed by
>> Hyper-V on AMD and is always used by Linux. The TLB flush required by
>> HV_X64_NESTED_ENLIGHTENED_TLB is much stricter than the local TLB flush
>> that TDP MMU wants to issue. We have also found that with TDP MMU L2 guest
>> boot performance on AMD is reproducibly slower compared to when TDP MMU is
>> disabled.
>>
>> Disable TDP MMU when using SVM Hyper-V for the time being while we
>> search for a better fix.
> 
> I'd suggest we go the other way around: disable
> HV_X64_NESTED_ENLIGHTENED_TLB on SVM:

Paolo suggested disabling TDP_MMU when HV_X64_NESTED_ENLIGHTENED_TLB is used, and
I prefer that option too. The enlighenment does offer a nice performance advantage
with non-TDP_MMU, and I did not see TDP_MMU perform any better compared to that.
Afaik the code to use the enlightenment on Hyper-V/SVM was written/tested before
TDP_MMU became the default.

If you have a specific scenario in mind, we could test and see what the implications
are there.

> 
> diff --git a/arch/x86/kvm/svm/svm_onhyperv.h b/arch/x86/kvm/svm/svm_onhyperv.h
> index 6981c1e9a809..be98da5a4277 100644
> --- a/arch/x86/kvm/svm/svm_onhyperv.h
> +++ b/arch/x86/kvm/svm/svm_onhyperv.h
> @@ -32,7 +32,8 @@ static inline void svm_hv_init_vmcb(struct vmcb *vmcb)
>  
>  static inline void svm_hv_hardware_setup(void)
>  {
> -       if (npt_enabled &&
> +       /* A comment about missing TLB flushes */
> +       if (!tdp_mmu_enabled && npt_enabled &&
>             ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB) {
>                 pr_info(KBUILD_MODNAME ": Hyper-V enlightened NPT TLB flush enabled\n");
>                 svm_x86_ops.tlb_remote_flush = hv_remote_flush_tlb;
> 
> this way we won't have a not-obvious-at-all MMU change on Hyper-V. I
> understand this may have some performance implications but MMU switch
> has some as well.
> 
>>
>> Link: https://lore.kernel.org/lkml/43980946-7bbf-dcef-7e40-af904c456250@xxxxxxxxxxxxxxxxxxx/t/#u
>> Signed-off-by: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx>
>> ---
>> Based on kvm-x86-mmu-6.3. The approach used here does not apply cleanly to
>> <=v6.2. This would be needed in stable too, and I don't know about putting
>> fixes tags.
> 
> Cc: stable@xxxxxxxxxxxxxxx # 5.17.0 
> 
> should do)
> 
>>
>> Jeremi
>>
>>  arch/x86/include/asm/kvm_host.h |  3 ++-
>>  arch/x86/kvm/mmu/mmu.c          |  5 +++--
>>  arch/x86/kvm/svm/svm.c          |  6 +++++-
>>  arch/x86/kvm/svm/svm_onhyperv.h | 10 ++++++++++
>>  arch/x86/kvm/vmx/vmx.c          |  3 ++-
>>  5 files changed, 22 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 4d2bc08794e4..a0868ae3688d 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -2031,7 +2031,8 @@ 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_forced_root_level,
>> -		       int tdp_max_root_level, int tdp_huge_page_level);
>> +		       int tdp_max_root_level, int tdp_huge_page_level,
>> +		       bool enable_tdp_mmu);
>>  
>>  static inline u16 kvm_read_ldt(void)
>>  {
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index c91ee2927dd7..5c0e28a7a3bc 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -5787,14 +5787,15 @@ void kvm_mmu_invpcid_gva(struct kvm_vcpu *vcpu, gva_t gva, unsigned long pcid)
>>  }
>>  
>>  void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
>> -		       int tdp_max_root_level, int tdp_huge_page_level)
>> +		       int tdp_max_root_level, int tdp_huge_page_level,
>> +		       bool enable_tdp_mmu)
>>  {
>>  	tdp_enabled = enable_tdp;
>>  	tdp_root_level = tdp_forced_root_level;
>>  	max_tdp_level = tdp_max_root_level;
>>  
>>  #ifdef CONFIG_X86_64
>> -	tdp_mmu_enabled = tdp_mmu_allowed && tdp_enabled;
>> +	tdp_mmu_enabled = tdp_mmu_allowed && tdp_enabled && enable_tdp_mmu;
>>  #endif
>>  	/*
>>  	 * max_huge_page_level reflects KVM's MMU capabilities irrespective
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index d13cf53e7390..070c3f7f8c9f 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -4925,6 +4925,7 @@ static __init int svm_hardware_setup(void)
>>  	struct page *iopm_pages;
>>  	void *iopm_va;
>>  	int r;
>> +	bool enable_tdp_mmu;
>>  	unsigned int order = get_order(IOPM_SIZE);
>>  
>>  	/*
>> @@ -4991,9 +4992,12 @@ static __init int svm_hardware_setup(void)
>>  	if (!boot_cpu_has(X86_FEATURE_NPT))
>>  		npt_enabled = false;
>>  
>> +	enable_tdp_mmu = svm_hv_enable_tdp_mmu();
>> +
>>  	/* Force VM NPT level equal to the host's paging level */
>>  	kvm_configure_mmu(npt_enabled, get_npt_level(),
>> -			  get_npt_level(), PG_LEVEL_1G);
>> +			  get_npt_level(), PG_LEVEL_1G,
>> +			  enable_tdp_mmu);
>>  	pr_info("Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
>>  
>>  	/* Setup shadow_me_value and shadow_me_mask */
>> diff --git a/arch/x86/kvm/svm/svm_onhyperv.h b/arch/x86/kvm/svm/svm_onhyperv.h
>> index 6981c1e9a809..aa49ac5d66bc 100644
>> --- a/arch/x86/kvm/svm/svm_onhyperv.h
>> +++ b/arch/x86/kvm/svm/svm_onhyperv.h
>> @@ -30,6 +30,11 @@ static inline void svm_hv_init_vmcb(struct vmcb *vmcb)
>>  		hve->hv_enlightenments_control.msr_bitmap = 1;
>>  }
>>  
>> +static inline bool svm_hv_enable_tdp_mmu(void)
>> +{
>> +	return !(npt_enabled && ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB);
>> +}
>> +
>>  static inline void svm_hv_hardware_setup(void)
>>  {
>>  	if (npt_enabled &&
>> @@ -84,6 +89,11 @@ static inline void svm_hv_init_vmcb(struct vmcb *vmcb)
>>  {
>>  }
>>  
>> +static inline bool svm_hv_enable_tdp_mmu(void)
>> +{
>> +	return true;
>> +}
>> +
>>  static inline void svm_hv_hardware_setup(void)
>>  {
>>  }
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index c788aa382611..4d3808755d39 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -8442,7 +8442,8 @@ static __init int hardware_setup(void)
>>  	vmx_setup_me_spte_mask();
>>  
>>  	kvm_configure_mmu(enable_ept, 0, vmx_get_max_tdp_level(),
>> -			  ept_caps_to_lpage_level(vmx_capability.ept));
>> +			  ept_caps_to_lpage_level(vmx_capability.ept),
>> +			  true);
>>  
>>  	/*
>>  	 * Only enable PML when hardware supports PML feature, and both EPT
>