linux-kernel - Re: [RFC PATCH v4 01/16] KVM: TDP_MMU: Go to next level if smaller private mapping exists

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Date:   Tue, 5 Sep 2023 16:10:33 +0800
From:   Binbin Wu <binbin.wu@...ux.intel.com>
To:     isaku.yamahata@...el.com, Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Zhi Wang <zhi.wang.linux@...il.com>, chen.bo@...el.com,
        hang.yuan@...el.com, tina.zhang@...el.com
Subject: Re: [RFC PATCH v4 01/16] KVM: TDP_MMU: Go to next level if smaller
 private mapping exists



On 7/26/2023 6:23 AM, isaku.yamahata@...el.com wrote:
> From: Xiaoyao Li <xiaoyao.li@...el.com>
>
> Cannot map a private page as large page if any smaller mapping exists.
>
> It has to wait for all the not-mapped smaller page to be mapped and
> promote it to larger mapping.
>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>   arch/x86/kvm/mmu/tdp_mmu.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 95ba78944712..a9f0f4ade2d0 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1293,7 +1293,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>   	tdp_mmu_for_each_pte(iter, mmu, is_private, raw_gfn, raw_gfn + 1) {
>   		int r;
>   
> -		if (fault->nx_huge_page_workaround_enabled)
> +		if (fault->nx_huge_page_workaround_enabled ||
> +		    kvm_gfn_shared_mask(vcpu->kvm))
>   			disallowed_hugepage_adjust(fault, iter.old_spte, iter.level);
>   
>   		/*
The implementation of disallowed_hugepage_adjust() is as following:

void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, 
int cur_level)
{
     if (cur_level > PG_LEVEL_4K &&
         cur_level == fault->goal_level &&
         is_shadow_present_pte(spte) &&
         !is_large_pte(spte) &&
         spte_to_child_sp(spte)->nx_huge_page_disallowed) {
             ...
     }
}

One condition is spte_to_child_sp(spte)->nx_huge_page_disallowed should be
true to decrease the goal level of the fault.
Does this condition make the change of this patch invalid?