Re: [PATCH 3/3] KVM: arm/arm64: vgic: Don't rely on the wrong pending table

From: Auger Eric <eric.auger@redhat.com>
To: Zenghui Yu <yuzenghui@huawei.com>,
	maz@kernel.org, james.morse@arm.com,
	julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com
Cc: wanghaibin.wang@huawei.com, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] KVM: arm/arm64: vgic: Don't rely on the wrong pending table
Date: Tue, 29 Oct 2019 13:17:00 +0100	[thread overview]
Message-ID: <5e4d1a2f-7107-efe3-9dde-626662e31ac5@redhat.com> (raw)
In-Reply-To: <20191029071919.177-4-yuzenghui@huawei.com>

Hi Zenghui, Marc,

On 10/29/19 8:19 AM, Zenghui Yu wrote:
> It's possible that two LPIs locate in the same "byte_offset" but target
> two different vcpus, where their pending status are indicated by two
> different pending tables.  In such a scenario, using last_byte_offset
> optimization will lead KVM relying on the wrong pending table entry.
> Let us use last_ptr instead, which can be treated as a byte index into
> a pending table and also, can be vcpu specific.
> 
> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
> ---
> 
> If this patch has done the right thing, we can even add the:
> 
> Fixes: 280771252c1b ("KVM: arm64: vgic-v3: KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES")
> 
> But to be honest, I'm not clear about what has this patch actually fixed.
> Pending tables should contain all zeros before we flush vgic_irq's pending
> status into guest's RAM (thinking that guest should never write anything
> into it). So the pending table entry we've read from the guest memory
> seems always be zero. And we will always do the right thing even if we
> rely on the wrong pending table entry.
> 
> I think I must have some misunderstanding here... Please fix me.
> 
>  virt/kvm/arm/vgic/vgic-v3.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
> index 5ef93e5041e1..7cd2e2f81513 100644
> --- a/virt/kvm/arm/vgic/vgic-v3.c
> +++ b/virt/kvm/arm/vgic/vgic-v3.c
> @@ -363,8 +363,8 @@ int vgic_v3_lpi_sync_pending_status(struct kvm *kvm, struct vgic_irq *irq)
>  int vgic_v3_save_pending_tables(struct kvm *kvm)
>  {
>  	struct vgic_dist *dist = &kvm->arch.vgic;
> -	int last_byte_offset = -1;
>  	struct vgic_irq *irq;
> +	gpa_t last_ptr = -1;
>  	int ret;
>  	u8 val;
>  
> @@ -384,11 +384,11 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
>  		bit_nr = irq->intid % BITS_PER_BYTE;
>  		ptr = pendbase + byte_offset;
>  
> -		if (byte_offset != last_byte_offset) {
> +		if (ptr != last_ptr) {
>  			ret = kvm_read_guest_lock(kvm, ptr, &val, 1);
>  			if (ret)
>  				return ret;
> -			last_byte_offset = byte_offset;
> +			last_ptr = ptr;
>  		}
>  
>  		stored = val & (1U << bit_nr);
> 
Acked-by: Eric Auger <eric.auger@redhat.com>

Thanks for fixing this.

Eric