linux-kernel - Re: [PATCH v18 049/121] KVM: x86/tdp_mmu: Apply mmu notifier callback to only shared GPA

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Date: Tue, 30 Jan 2024 15:19:37 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: isaku.yamahata@...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>, Kai Huang <kai.huang@...el.com>,
 chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com
Subject: Re: [PATCH v18 049/121] KVM: x86/tdp_mmu: Apply mmu notifier callback
 to only shared GPA



On 1/23/2024 7:53 AM, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
>
> The private GPAs that typically guest memfd backs aren't subject to MMU
> notifier because it isn't mapped into virtual address of user process.
> kvm_tdp_mmu_handle_gfn() handles the callback of the MMU notifier,
> clear_flush_young(), clear_young(), test_gfn() and change_pte().  Make

Nit: test_gfn() -> test_young(), and a same typo in the comment below.

Reviewed-by: Binbin Wu <binbin.wu@...ux.intel.com>

> kvm_tdp_mmu_handle_gfn() aware of private mapping and skip private mapping.
>
> Even with AS_UNMOVABLE set, those mmu notifier are called.  For example,
> ksmd triggers change_pte().
>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
> v18:
> - newly added
> ---
>   arch/x86/kvm/mmu/tdp_mmu.c | 22 +++++++++++++++++++++-
>   1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index e7514a807134..fdc6e2221c33 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1157,9 +1157,29 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
>   	 * into this helper allow blocking; it'd be dead, wasteful code.
>   	 */
>   	for_each_tdp_mmu_root(kvm, root, range->slot->as_id) {
> +		gfn_t start, end;
> +
> +		/*
> +		 * This function is called on behalf of mmu_notifier of
> +		 * clear_flush_young(), clear_young(), test_gfn(), and
test_gfn() should be test_young()

> +		 * change_pte().  They apply to only shared GPAs.
> +		 */
> +		WARN_ON_ONCE(range->only_private);
> +		WARN_ON_ONCE(!range->only_shared);
> +		if (is_private_sp(root))
> +			continue;
> +
> +		/*
> +		 * For TDX shared mapping, set GFN shared bit to the range,
> +		 * so the handler() doesn't need to set it, to avoid duplicated
> +		 * code in multiple handler()s.
> +		 */
> +		start = kvm_gfn_to_shared(kvm, range->start);
> +		end = kvm_gfn_to_shared(kvm, range->end);
> +
>   		rcu_read_lock();
>   
> -		tdp_root_for_each_leaf_pte(iter, root, range->start, range->end)
> +		tdp_root_for_each_leaf_pte(iter, root, start, end)
>   			ret |= handler(kvm, &iter, range);
>   
>   		rcu_read_unlock();