From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH 8/8] KVM: MMU: Move free_zapped_mmu_pages() out of the
 protection of mmu_lock
Date: Mon, 4 Feb 2013 11:50:00 -0200
Message-ID: <20130204135000.GC9005@amt.cnet>
References: <20130123191231.d66489d2.yoshikawa_takuya_b1@lab.ntt.co.jp>
 <20130123191811.efba4200.yoshikawa_takuya_b1@lab.ntt.co.jp>
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Cc: gleb@redhat.com, kvm@vger.kernel.org
To: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
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On Wed, Jan 23, 2013 at 07:18:11PM +0900, Takuya Yoshikawa wrote:
> We noticed that kvm_mmu_zap_all() could take hundreds of milliseconds
> for zapping mmu pages with mmu_lock held.
> 
> Although we need to do conditional rescheduling for completely
> fixing this issue, we can reduce the hold time to some extent by moving
> free_zapped_mmu_pages() out of the protection.  Since invalid_list can
> be very long, the effect is not negligible.
> 
> Note: this patch does not treat non-trivial cases.
> 
> Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>

Can you describe the case thats biting? Is it

        /*
         * If memory slot is created, or moved, we need to clear all
         * mmio sptes.
         */
        if (npages && old.base_gfn != mem->guest_phys_addr >> PAGE_SHIFT) {
                kvm_mmu_zap_all(kvm);
                kvm_reload_remote_mmus(kvm);
        }

Because conditional rescheduling for kvm_mmu_zap_all() might not be
desirable: KVM_SET_USER_MEMORY has low latency requirements.