linux-kernel - Re: [PATCH v2 1/4] iommu/vt-d: Check before setting PGSNP bit in pasid table entry

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Message-ID: <de836242-e188-7798-f779-69703eebd45a@linux.intel.com>
Date:   Sun, 24 Apr 2022 14:23:38 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] iommu/vt-d: Check before setting PGSNP bit in
 pasid table entry

On 2022/4/24 13:55, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Sunday, April 24, 2022 12:38 PM
>>
>> On 2022/4/24 11:37, Tian, Kevin wrote:
>>>>> This should be rebased on top of Jason's enforce coherency series
>>>>> instead of blindly setting it. No matter whether it's legacy mode
>>>>> where we set SNP in PTE or scalable mode where we set PGSNP
>>>>> in PASID entry for entire page table, the trigger point should be
>>>>> same i.e. when someone calls enforce_cache_coherency().
>>>> With Jason's enforce coherency series merged, we even don't need to set
>>>> PGSNP bit of a pasid entry for second level translation. 2nd level
>>>> always supports SNP in PTEs, so set PGSNP in pasid table entry is
>>>> unnecessary.
>>>>
>>> Yes, this sounds correct for 2nd-level.
>>>
>>> but setting PGSNP of 1st level translation is also relevant to that
>>> change when talking about enforcing coherency in the guest. In
>>> this case PASID_FLAG_PAGE_SNOOP should be set also after
>>> enforce_cache_coherency() is called.
>>
>> Yes. Agreed.
>>
>>> Currently it's always set for unmanaged domain in
>>> domain_setup_first_level():
>>>
>>> 	if (domain->domain.type == IOMMU_DOMAIN_UNMANAGED)
>>> 		flags |= PASID_FLAG_PAGE_SNOOP;
>>>
>>> Suppose we need a separate interface to update PGSNP after pasid
>>> entry is set up.
>>
>> Currently enforcing coherency is only used in VFIO. In the VFIO use
>> case, it's safe to always set PGSNP when an UNMANAGED domain is attached
>> on the first level pasid translation. We could add support of updating
>> PGSNP after pasid entry setup once there's a real need.
>>
> 
> The real need is here. The iommu driver should not assume the
> policy of VFIO, which is already communicated via the new
> enforce_cache_coherency() interface. The same policy should
> apply no matter whether 1st or 2nd level is in-use.

Okay, I think I will move this patch out of this series and put it in a
separated one for VT-d improvements after Jason's enforcing snoop series
gets merged. Thanks for your suggestions.

Best regards,
baolu