linux-kernel - Re: [PATCH 05/12] iommufd/hw_pagetable: Do not populate user-managed hw

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Date:   Fri, 10 Mar 2023 20:51:48 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Nicolin Chen <nicolinc@...dia.com>, Yi Liu <yi.l.liu@...el.com>
Cc:     baolu.lu@...ux.intel.com, joro@...tes.org,
        alex.williamson@...hat.com, jgg@...dia.com, kevin.tian@...el.com,
        robin.murphy@....com, cohuck@...hat.com, eric.auger@...hat.com,
        kvm@...r.kernel.org, mjrosato@...ux.ibm.com,
        chao.p.peng@...ux.intel.com, yi.y.sun@...ux.intel.com,
        peterx@...hat.com, jasowang@...hat.com,
        shameerali.kolothum.thodi@...wei.com, lulu@...hat.com,
        suravee.suthikulpanit@....com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 05/12] iommufd/hw_pagetable: Do not populate user-managed
 hw_pagetables

On 2023/3/10 14:50, Nicolin Chen wrote:
> On Fri, Mar 10, 2023 at 10:25:10AM +0800, Baolu Lu wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> On 3/9/23 4:09 PM, Yi Liu wrote:
>>> From: Nicolin Chen <nicolinc@...dia.com>
>>>
>>> A user-managed hw_pagetable does not need to get populated, since it is
>>> managed by a guest OS. Move the iopt_table_add_domain and list_add_tail
>>> calls into a helper, where the hwpt pointer will be redirected to its
>>> hwpt->parent if it's available.
>>>
>>> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
>>> Signed-off-by: Yi Liu <yi.l.liu@...el.com>
>>> ---
>>>    drivers/iommu/iommufd/hw_pagetable.c | 20 ++++++++++++++++++--
>>>    1 file changed, 18 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/iommu/iommufd/hw_pagetable.c b/drivers/iommu/iommufd/hw_pagetable.c
>>> index 16e92a1c150b..6e45ec0a66fa 100644
>>> --- a/drivers/iommu/iommufd/hw_pagetable.c
>>> +++ b/drivers/iommu/iommufd/hw_pagetable.c
>>> @@ -43,6 +43,23 @@ int iommufd_hw_pagetable_enforce_cc(struct iommufd_hw_pagetable *hwpt)
>>>        return 0;
>>>    }
>>>
>>> +static int iommufd_hw_pagetable_link_ioas(struct iommufd_hw_pagetable *hwpt)
>>> +{
>>> +     int rc;
>>> +
>>> +     if (hwpt->parent)
>>> +             hwpt = hwpt->parent;
>>> +
>>> +     if (!list_empty(&hwpt->hwpt_item))
>>> +             return 0;
>>
>> What is above check for? Is it "the hwpt has already been inserted into
>> the hwpt list of its ioas in another place"?
>>
>> If so, is it possible that hwpt will be deleted from the list even when
>> this user hwpt is still linked to the ioas?
> 
> It means that the hwpt is already linked to the ioas. And the
> hwpt_item can be only empty after a destroy().
> 
> With that being said, after I think it through, perhaps Yi's
> previous change removing it might be better. So, it could be:
> 
> -------------------------------------------------------------------------------
> +	/*
> +	 * Only a parent hwpt needs to be linked to the IOAS. And a hwpt->parent
> +	 * must be linked to the IOAS already, when it's being allocated.
> +	 */
>   	if (hwpt->parent)
> -		hwpt = hwpt->parent;
> -
> -	if (!list_empty(&hwpt->hwpt_item))
>   		return 0;
>   
> -------------------------------------------------------------------------------
> 
> I was concerned about the case where a device gets attached to
> the nested hwpt without staging at the parent hwpt first. But,
> the link between the parent hwpt and the IOAS happened inside
> the allocation function now, not attach() any more.

Yes, it's clearer.

Best regards,
baolu