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

Sean Christopherson
2021-08-05T21:51:50Z

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@amd.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;

/*