linux-kernel - Re: [PATCH V3 02/12] KVM: X86/MMU: Add using_local_root

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Date:   Thu, 26 May 2022 21:28:50 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>
Subject: Re: [PATCH V3 02/12] KVM: X86/MMU: Add using_local_root_page()

On Sat, May 21, 2022 at 09:16:50PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@...group.com>
> 
> In some cases, local root pages are used for MMU.  It is often using
> to_shadow_page(mmu->root.hpa) to check if local root pages are used.
> 
> Add using_local_root_page() to directly check if local root pages are
> used or needed to be used even mmu->root.hpa is not set.
> 
> Prepare for making to_shadow_page(mmu->root.hpa) returns non-NULL via
> using local shadow [root] pages.
> 
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 40 +++++++++++++++++++++++++++++++++++++---
>  1 file changed, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index efe5a3dca1e0..624b6d2473f7 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1690,6 +1690,39 @@ static void drop_parent_pte(struct kvm_mmu_page *sp,
>  	mmu_spte_clear_no_track(parent_pte);
>  }
>  
> +/*
> + * KVM uses the VCPU's local root page (vcpu->mmu->pae_root) when either the
> + * shadow pagetable is using PAE paging or the host is shadowing nested NPT for
> + * 32bit L1 hypervisor.

How about using the terms "private" and "shared" instead of "local" and
"non-local"? I think that more accurately conveys what is special about
these pages: they are private to the vCPU using them. And then "shared"
is more intuitive to understand than "non-local" (which is used
elsewhere in this series).

> + *
> + * It includes cases:
> + *	nonpaging when !tdp_enabled				(direct paging)
> + *	shadow paging for 32 bit guest when !tdp_enabled	(shadow paging)
> + *	NPT in 32bit host (not shadowing nested NPT)		(direct paging)
> + *	shadow nested NPT for 32bit L1 hypervisor in 32bit host (shadow paging)
> + *	shadow nested NPT for 32bit L1 hypervisor in 64bit host (shadow paging)
> + *
> + * For the first four cases, mmu->root_role.level is PT32E_ROOT_LEVEL and the
> + * shadow pagetable is using PAE paging.
> + *
> + * For the last case, it is
> + * 	mmu->root_role.level > PT32E_ROOT_LEVEL &&
> + * 	!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL
> + * And if this condition is true, it must be the last case.
> + *
> + * With the two conditions combined, the checking condition is:
> + * 	mmu->root_role.level == PT32E_ROOT_LEVEL ||
> + * 	(!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL)
> + *
> + * (There is no "mmu->root_role.level > PT32E_ROOT_LEVEL" here, because it is
> + *  already ensured that mmu->root_role.level >= PT32E_ROOT_LEVEL)
> + */
> +static bool using_local_root_page(struct kvm_mmu *mmu)
> +{
> +	return mmu->root_role.level == PT32E_ROOT_LEVEL ||
> +	       (!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL);
> +}
> +
>  static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu, int direct)
>  {
>  	struct kvm_mmu_page *sp;
> @@ -4252,10 +4285,11 @@ static bool fast_pgd_switch(struct kvm *kvm, struct kvm_mmu *mmu,
>  {
>  	/*
>  	 * For now, limit the caching to 64-bit hosts+VMs in order to avoid
> -	 * having to deal with PDPTEs. We may add support for 32-bit hosts/VMs
> -	 * later if necessary.
> +	 * having to deal with PDPTEs.  Local roots can not be put into
> +	 * mmu->prev_roots[] because mmu->pae_root can not be shared for
> +	 * different roots at the same time.

This comment ends up being a little confusing by the end of this series
because using_local_root_page() does not necessarily imply pae_root is
in use. i.e. case 5 (shadow nested NPT for 32bit L1 hypervisor in 64bit
host) does not use pae_root.

How about rewording this commit to say something like:

  If the vCPU is using a private root, it might be using pae_root, which
  cannot be shared for different roots at the same time.

>  	 */
> -	if (VALID_PAGE(mmu->root.hpa) && !to_shadow_page(mmu->root.hpa))
> +	if (unlikely(using_local_root_page(mmu)))
>  		kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);
>  
>  	if (VALID_PAGE(mmu->root.hpa))
> -- 
> 2.19.1.6.gb485710b
>