linux-kernel - Re: Question about domain

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Date:   Fri, 27 Nov 2020 10:12:17 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jerry Snitselaar <jsnitsel@...hat.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        stable@...nel.vger.org
Subject: Re: Question about domain_init (v5.3-v5.7)

Hi Jerry,

On 11/27/20 5:35 AM, Jerry Snitselaar wrote:
> 
> Lu Baolu @ 2020-11-26 04:01 MST:
> 
>> Hi Jerry,
>>
>> On 2020/11/26 4:27, Jerry Snitselaar wrote:
>>> Is there a reason we check the requested guest address width against
>>> the
>>> iommu's mgaw, instead of the agaw that we already know for the iommu?
>>> I've run into a case with a new system where the mgaw reported is 57,
>>> but if they set PAE to 46 instead of 52 in the bios, then sagaw reports
>>> the highest supported agaw is 48 and the domain_init code fails here. In
>>
>> Isn't this a platform bug? If it's too late to fix it in the BIOS, you
>> maybe have to add a platform specific quirk to set mgaw to the highest
>> supported agaw?
>>
>> Best regards,
>> baolu
> 
> Is there somewhere you can point me to that discusses how they should be
> setting the mgaw? I misunderstood when I previously asked you about
> whether the mgaw could be a value that was greater than any of sagaw.
> If it is a bios issue, then they should fix it there.

MGAW indicates the max gpa width supported by 2nd translation. The VT-d
spec requires that this value must be at least equal to the host
physical addressibility. According to this, BIOS is good, right?

For this failure case, domain_init() just wants to find a suitable agaw
for the private domain. I think it makes sense to check against
iommu->agaw instead of cap_mgaw.

Best regards,
baolu

> 
>>
>>> other places like prepare_domain_attach_device, the dmar domain agaw
>>> gets adjusted down to the iommu agaw. The agaw of the iommu gets
>>> determined based off what is reported for sagaw. I'm wondering if it
>>> can't instead do:
>>> ---
>>>    drivers/iommu/intel-iommu.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>> diff --git a/drivers/iommu/intel-iommu.c
>>> b/drivers/iommu/intel-iommu.c
>>> index 6ca5c92ef2e5..a8e41ec36d9e 100644
>>> --- a/drivers/iommu/intel-iommu.c
>>> +++ b/drivers/iommu/intel-iommu.c
>>> @@ -1862,8 +1862,8 @@ static int domain_init(struct dmar_domain *domain, struct intel_iommu *iommu,
>>>    	domain_reserve_special_ranges(domain);
>>>    	/* calculate AGAW */
>>> -	if (guest_width > cap_mgaw(iommu->cap))
>>> -	        guest_width = cap_mgaw(iommu->cap);
>>> +	if (guest_width > agaw_to_width(iommu->agaw))
>>> +	        guest_width = agaw_to_width(iommu->agaw);
>>>    	domain->gaw = guest_width;
>>>    	adjust_width = guestwidth_to_adjustwidth(guest_width);
>>>    	agaw = width_to_agaw(adjust_width);
>>> --
>>> 2.27.0
>>>
>>> Thoughts? With the former code the ehci device for the ilo fails when
>>> trying to get a private domain.
>>> Thanks,
>>> Jerry
>>>
>