Re: [PATCH] KVM: arm/arm64: vgic: Handle GICR_PENDBASER.PTZ filed as RAZ

From: Zenghui Yu <yuzenghui@huawei.com>
To: Auger Eric <eric.auger@redhat.com>, Marc Zyngier <maz@kernel.org>
Cc: <andre.przywara@arm.com>, <linux-kernel@vger.kernel.org>,
	<wanghaibin.wang@huawei.com>, <kvmarm@lists.cs.columbia.edu>,
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] KVM: arm/arm64: vgic: Handle GICR_PENDBASER.PTZ filed as RAZ
Date: Tue, 24 Dec 2019 14:14:33 +0800	[thread overview]
Message-ID: <5df2ebf7-f1e0-5d55-cdae-15b2fd1675d6@huawei.com> (raw)
In-Reply-To: <12a1e25b-617d-6b04-6a5a-4c67a39565a5@redhat.com>

On 2019/12/24 12:45, Auger Eric wrote:
> Hi Zenghui,
> 
> On 12/24/19 3:52 AM, Zenghui Yu wrote:
>> Hi Marc, Eric,
>>
>> On 2019/12/23 22:07, Marc Zyngier wrote:
>>> Hi Zenghui,
>>>
>>> On 2019-12-23 13:43, Zenghui Yu wrote:
>>>> I noticed there is no userspace access callbacks for GICR_PENDBASER,
>>>> so this patch will make the PTZ field also 'Read As Zero' by userspace.
>>>> Should we consider adding a uaccess_read callback for GICR_PENDBASER
>>>> which just returns the unchanged vgic_cpu->pendbaser to userspace?
>>>> (Though this is really not a big deal. We now always emulate the PTZ
>>>> field to guest as RAZ. And 'vgic_cpu->pendbaser & GICR_PENDBASER_PTZ'
>>>> only indicates whether KVM will optimize the LPI enabling process,
>>>> where Read As Zero indicates never optimize..)
>>>
>>> I don't think adding a userspace accessor would help much. All this
>>> bit tells userspace is that the guest has programmed a zero filled
>>> table. On restore, we'd avoid a rescan of the table if there was
>>> no LPI mapped.
>>
>> Yes, I agree.
>>
>>> And thinking of it, this fixes a bug for non-Linux guests: If you write
>>> PTZ=1, we never clear it. Which means that if userspace saves and
>>> restores
>>> PENDBASER with PTZ set, we'll never restore the pending bits, which is
>>> pretty bad (see vgic_enable_lpis()).
>>
>> But I'm afraid I can't follow this point. After reading the code (with
>> Qemu) a bit further, the Redistributors are restored before the ITS.
> 
> This is also part of the kernel documentation:
> Documentation/virt/kvm/devices/arm-vgic-its.txt (ITS restore sequence)

Yeah, I see. Thanks for the pointer, Eric!

Zenghui

>   So
>> there should be _no_ LPI has been mapped when we're restoring GICR_CTLR
>> and enabling LPI, which says we will not scan the whole pending table
>> and restore pending by vgic_enable_lpis()/its_sync_lpi_pending_table(),
>> regardless of what the PTZ is.
>>
>> Instead, vgic_its_restore_ite()/vgic_v3_lpi_sync_pending_status() is
>> where we actually read the guest RAM and restore the LPI pending state.
> yes the pending state is restored from
> vgic_its_restore_ite/vgic_add_lpi/vgic_v3_lpi_sync_pending_status and
> this path ignores the PTZ.
> 
> Thanks
> 
> Eric
>> Which means we will still do the right thing even for non-Linux guests.
>> Not sure if I've got things correctly here.
>>
>> In the end, let's keep the patch as it is.
>>
>>>
>>> This patch on its own fixes more than one bug!
>>>
>>
>> If so, just by luck ;-)