linux-kernel - Re: [PATCH 2/5] KVM: MMU: simplify mmu_set

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Message-ID: <20121112231223.GC5798@amt.cnet>
Date:	Mon, 12 Nov 2012 21:12:23 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/5] KVM: MMU: simplify mmu_set_spte

On Mon, Nov 05, 2012 at 08:10:08PM +0800, Xiao Guangrong wrote:
> In order to detecting spte remapping, we can simply check whether the
> spte has already been pointing to the pfn even if the spte is not the
> last spte for middle spte is pointing to the kernel pfn which can not
> be mapped to userspace
> 
> Also, update slot and stat.lpages iff the spte is not remapped
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  arch/x86/kvm/mmu.c |   40 +++++++++++++---------------------------
>  1 files changed, 13 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 692ebb1..4ea731e 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2420,8 +2420,7 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  			 pfn_t pfn, bool speculative,
>  			 bool host_writable)
>  {
> -	int was_rmapped = 0;
> -	int rmap_count;
> +	bool was_rmapped = false;
> 
>  	pgprintk("%s: spte %llx access %x write_fault %d"
>  		 " user_fault %d gfn %llx\n",
> @@ -2429,25 +2428,13 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  		 write_fault, user_fault, gfn);
> 
>  	if (is_rmap_spte(*sptep)) {
> -		/*
> -		 * If we overwrite a PTE page pointer with a 2MB PMD, unlink
> -		 * the parent of the now unreachable PTE.
> -		 */
> -		if (level > PT_PAGE_TABLE_LEVEL &&
> -		    !is_large_pte(*sptep)) {
> -			struct kvm_mmu_page *child;
> -			u64 pte = *sptep;
> +		if (pfn != spte_to_pfn(*sptep)) {
> +			struct kvm_mmu_page *sp = page_header(__pa(sptep));
> 
> -			child = page_header(pte & PT64_BASE_ADDR_MASK);
> -			drop_parent_pte(child, sptep);
> -			kvm_flush_remote_tlbs(vcpu->kvm);

How come its safe to drop this case?

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