linux-kernel - Re: [PATCH] KVM: x86/MMU: Zap all when removing memslot if VM has assigned device

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Date:   Fri, 16 Aug 2019 09:16:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alex Willamson <alex.williamson@...hat.com>
Subject: Re: [PATCH] KVM: x86/MMU: Zap all when removing memslot if VM has
 assigned device

On 15/08/19 17:12, Sean Christopherson wrote:
> Alex Williamson reported regressions with device assignment when KVM
> changed its memslot removal logic to zap only the SPTEs for the memslot
> being removed.  The source of the bug is unknown at this time, and root
> causing the issue will likely be a slow process.  In the short term, fix
> the regression by zapping all SPTEs when removing a memslot from a VM
> with assigned device(s).
> 
> Fixes: 4e103134b862 ("KVM: x86/mmu: Zap only the relevant pages when removing a memslot", 2019-02-05)
> Reported-by: Alex Willamson <alex.williamson@...hat.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
> 
> An alternative idea to a full revert.  I assume this would be easy to
> backport, and also easy to revert or quirk depending on where the bug
> is hiding.

We're not sure that it only happens with assigned devices; it's just
that assigned BARs are the memslots that are more likely to be
reprogrammed at boot.  So this patch feels unsafe.

Paolo

> 
>  arch/x86/kvm/mmu.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 8f72526e2f68..358b93882ac6 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -5659,6 +5659,17 @@ static void kvm_mmu_invalidate_zap_pages_in_memslot(struct kvm *kvm,
>  	bool flush;
>  	gfn_t gfn;
>  
> +	/*
> +	 * Zapping only the removed memslot introduced regressions for VMs with
> +	 * assigned devices.  It is unknown what piece of code is buggy.  Until
> +	 * the source of the bug is identified, zap everything if the VM has an
> +	 * assigned device.
> +	 */
> +	if (kvm_arch_has_assigned_device(kvm)) {
> +		kvm_mmu_zap_all(kvm);
> +		return;
> +	}
> +
>  	spin_lock(&kvm->mmu_lock);
>  
>  	if (list_empty(&kvm->arch.active_mmu_pages))
>