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

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Date:   Tue, 29 Oct 2019 20:30:32 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Auger Eric <eric.auger@...hat.com>, <maz@...nel.org>,
        <james.morse@....com>, <julien.thierry.kdev@...il.com>,
        <suzuki.poulose@....com>
CC:     <wanghaibin.wang@...wei.com>, <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] KVM: arm/arm64: vgic: Don't rely on the wrong pending
 table

On 2019/10/29 20:17, Auger Eric wrote:
> 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@...wei.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@...hat.com>

Thanks Eric,


Zenghui