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Subject: Re: [PATCH] KVM: arm/arm64: vgic: Handle GICR_PENDBASER.PTZ filed as
 RAZ
To:     Marc Zyngier <maz@kernel.org>, <eric.auger@redhat.com>
CC:     <andre.przywara@arm.com>, <linux-arm-kernel@lists.infradead.org>,
        <kvmarm@lists.cs.columbia.edu>, <linux-kernel@vger.kernel.org>,
        <wanghaibin.wang@huawei.com>
References: <20191220111833.1422-1-yuzenghui@huawei.com>
 <71e3dcc00ad5ab8dffd732bfe7381705@www.loen.fr>
From:   Zenghui Yu <yuzenghui@huawei.com>
Message-ID: <5f5f499a-d025-cc9f-66f3-37fe958df246@huawei.com>
Date:   Mon, 23 Dec 2019 14:50:51 +0800
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Hi Marc, Eric,

On 2019/12/20 21:07, Marc Zyngier wrote:
> On 2019-12-20 11:18, Zenghui Yu wrote:
>> Although guest will hardly read and use the PTZ (Pending Table Zero)
>> bit in GICR_PENDBASER, let us emulate the architecture strictly.
>> As per IHI 0069E 9.11.30, PTZ field is WO, and reads as 0.
>>
>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
>> ---
>>
>> Noticed when checking all fields of GICR_PENDBASER register.
>> But _not_ sure whether it's worth a fix, as Linux never sets
>> the PTZ bit before enabling LPI (set GICR_CTLR_ENABLE_LPIS).
>>
>> And I wonder under which scenarios can this bit be written as 1.
>> It seems difficult for software to determine whether the pending
>> table contains all zeros when writing this bit.
> 
> This is a useless HW optimization, where it can avoid reading the
> pending table the very first time you write to this register if
> it is told that it is all zero. A decent ITS implementation
> already has a mechanism to find out about the pending bits by
> looking into the IMPDEF area (the first 1kB) of the pending table.

Yeah, AFAICT this is what Hisilicon has already implemented today.

> PTZ is just yet another way to do the same thing.
> 
> This can only happen once in the lifetime of the system (when allocating
> the table), and Linux doesn't really care.

I now get it, thanks for teaching me that!

> As usual, the GIC is setting
> the level of useless complexity pretty high...
> 
>>
>>  virt/kvm/arm/vgic/vgic-mmio-v3.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> index 7dfd15dbb308..ebc218840fc2 100644
>> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> @@ -414,8 +414,11 @@ static unsigned long
>> vgic_mmio_read_pendbase(struct kvm_vcpu *vcpu,
>>                           gpa_t addr, unsigned int len)
>>  {
>>      struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>> +    u64 value = vgic_cpu->pendbaser;
>>
>> -    return extract_bytes(vgic_cpu->pendbaser, addr & 7, len);
>> +    value &= ~GICR_PENDBASER_PTZ;
>> +
>> +    return extract_bytes(value, addr & 7, len);
>>  }
>>
>>  static void vgic_mmio_write_pendbase(struct kvm_vcpu *vcpu,
> 
> Otherwise looks good. I'll queue it with Eric's correction
> to the subject line.

Thanks both and Merry Christmas!

Zenghui