linux-kernel - Re: [PATCH] KVM: x86: Manually flush collapsible SPTEs only when toggling flags

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Message-ID: <47331a39-7f74-661c-b248-266eec420efa@redhat.com>
Date:   Tue, 17 Sep 2019 15:33:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Manually flush collapsible SPTEs only when
 toggling flags

On 11/09/19 21:19, Sean Christopherson wrote:
> Zapping collapsible sptes, a.k.a. 4k sptes that can be promoted into a
> large page, is only necessary when changing only the dirty logging flag
> of a memory region.  If the memslot is also being moved, then all sptes
> for the memslot are zapped when it is invalidated.  When a memslot is
> being created, it is impossible for there to be existing dirty mappings,
> e.g. KVM can have MMIO sptes, but not present, and thus dirty, sptes.
> 
> Note, the comment and logic are shamelessly borrowed from MIPS's version
> of kvm_arch_commit_memory_region().
> 
> Fixes: 3ea3b7fa9af06 ("kvm: mmu: lazy collapse small sptes into large sptes")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/x86.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b4cfd786d0b6..70e82e8f5c41 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9635,8 +9635,13 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  	 * Scan sptes if dirty logging has been stopped, dropping those
>  	 * which can be collapsed into a single large-page spte.  Later
>  	 * page faults will create the large-page sptes.
> +	 *
> +	 * There is no need to do this in any of the following cases:
> +	 * CREATE:	No dirty mappings will already exist.
> +	 * MOVE/DELETE:	The old mappings will already have been cleaned up by
> +	 *		kvm_arch_flush_shadow_memslot()
>  	 */
> -	if ((change != KVM_MR_DELETE) &&
> +	if (change == KVM_MR_FLAGS_ONLY &&
>  		(old->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
>  		!(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
>  		kvm_mmu_zap_collapsible_sptes(kvm, new);
> 

Queued, thanks.

Paolo