linux-kernel - Re: [PATCH v1 1/3] KVM: x86: Convert TDP level calculation to vendor's specific code

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Message-ID: <YQxdbq+yoTIJmpL+@google.com>
Date:   Thu, 5 Aug 2021 21:51:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wei Huang <wei.huang2@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v1 1/3] KVM: x86: Convert TDP level calculation to
 vendor's specific code

On Thu, Aug 05, 2021, Wei Huang wrote:
> Currently the TDP level for x86 vCPU is calculated by checking both
> MAXPHYADDR and max_tdp_level. This design assumes that all x86 CPUs have
> the flexibility of changing the nested page table level different from host
> CPU. This assumption might not be true.

Heh, no need to be circumspect, just state that 5-level NPT inherits CR4.LA57
from the host.  I didn't fully understand this sentence until I looked at patch 3.

> To solve this problem, let us
> create a kvm_x86_ops specific function for TDP level calculation.
> 
> Signed-off-by: Wei Huang <wei.huang2@....com>
> ---

...

> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 974cbfb1eefe..20ddfbac966e 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;

Ha, this is leftover crud that can get zapped no matter what.

>  	/* emulate context */
>  

...

> -static inline int kvm_mmu_get_tdp_level(struct kvm_vcpu *vcpu)
> -{
> -	/* Use 5-level TDP if and only if it's useful/necessary. */
> -	if (max_tdp_level == 5 && cpuid_maxphyaddr(vcpu) <= 48)

I'd strongly prefer to keep this logic in the MMU.  When this was in vendor code,
there were multiple bugs where the MMU and VMX didn't communicate correctly, I
really don't want to back down that road.

Actually, I'm very, very tempted to say we should simply drop the cpuid_maxphyaddr()
bit and just return the max level (and I suppose rename it), e.g.

	return mmu_tdp_level;

It's effectively a single 4kb page per VM, and Intel's numbers on 5-level paging
were that there was no measurable cost to the extra level.  I would hope that
holds true here, too.

If we want to keep the MAXPHYADDR behavior, I'd vote for something like:

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index b4b65c21b2ca..7e35f2bf89b4 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;
 static int max_tdp_level __read_mostly;

 enum {
@@ -4645,6 +4646,9 @@ 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)
 {
+       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;
@@ -5336,10 +5340,11 @@ 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;

        /*