linux-kernel - Re: [PATCH] KVM: x86/mmu: optimizing the code in mmu_try_to_unsync

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Date:   Fri, 20 May 2022 17:54:28 +0800
From:   Yuan Yao <yuan.yao@...ux.intel.com>
To:     Yun Lu <luyun_611@....com>
Cc:     pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: optimizing the code in
 mmu_try_to_unsync_pages

On Fri, May 20, 2022 at 02:09:07PM +0800, Yun Lu wrote:
> There is no need to check can_unsync and prefetch in the loop
> every time, just move this check before the loop.
>
> Signed-off-by: Yun Lu <luyun@...inos.cn>
> ---
>  arch/x86/kvm/mmu/mmu.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 311e4e1d7870..e51e7735adca 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2534,6 +2534,12 @@ int mmu_try_to_unsync_pages(struct kvm *kvm, const struct kvm_memory_slot *slot,
>  	if (kvm_slot_page_track_is_active(kvm, slot, gfn, KVM_PAGE_TRACK_WRITE))
>  		return -EPERM;
>
> +	if (!can_unsync)
> +		return -EPERM;
> +
> +	if (prefetch)
> +		return -EEXIST;
> +
>  	/*
>  	 * The page is not write-tracked, mark existing shadow pages unsync
>  	 * unless KVM is synchronizing an unsync SP (can_unsync = false).  In
> @@ -2541,15 +2547,9 @@ int mmu_try_to_unsync_pages(struct kvm *kvm, const struct kvm_memory_slot *slot,
>  	 * allowing shadow pages to become unsync (writable by the guest).
>  	 */
>  	for_each_gfn_indirect_valid_sp(kvm, sp, gfn) {
> -		if (!can_unsync)
> -			return -EPERM;
> -
>  		if (sp->unsync)
>  			continue;
>
> -		if (prefetch)
> -			return -EEXIST;
> -

Consider the case that for_each_gfn_indirect_valid_sp() loop is
not triggered, means the gfn is not MMU page table page:

The old behavior when : return 0;
The new behavior with this change: returrn -EPERM / -EEXIST;

It at least breaks FNAME(sync_page) -> make_spte(prefetch = true, can_unsync = false)
which removes PT_WRITABLE_MASK from last level mapping unexpectedly.


>  		/*
>  		 * TDP MMU page faults require an additional spinlock as they
>  		 * run with mmu_lock held for read, not write, and the unsync
> --
> 2.25.1
>
>
> No virus found
> 		Checked by Hillstone Network AntiVirus
>