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

From: Lu Baolu
Date: Thu Nov 26 2020 - 21:19:54 EST


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