Re: Question about domain_init (v5.3-v5.7)

[Jerry Snitselaar <jsnitsel@redhat.com>]

Jerry Snitselaar @ 2020-11-30 10:50 MST:

> Lu Baolu @ 2020-11-26 19:12 MST:
>
>> 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
>>
>
> From this bit in the spec about MGAW:
>
>     Guest addressability for a given DMA request is limited to the
>     minimum of the value reported through this field and the adjusted
>     guest address width of the corresponding page-table structure.
>     (Adjusted guest address widths supported by hardware are reported
>     through the SAGAW field).
>
> That does suggest it should be adjusted down to the sagaw value in this case, yes?
> Just want to make sure I'm understanding it correctly.

Or I guess that is really talking about if you had an mgaw lower than the the
sagaw, the dma request would be limited to that lower mgaw value.

>
>>> 
>>>>
>>>>> 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
>>>>>
>>>