linux-kernel - RE: [PATCH 1/1] iommu/vt-d: Correctly check format of page table in debugfs

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Date:   Fri, 26 Jul 2019 17:17:20 +0000
From:   "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: RE: [PATCH 1/1] iommu/vt-d: Correctly check format of page table in
 debugfs

> On 7/22/19 1:21 PM, Prakhya, Sai Praneeth wrote:
> > Hi Allen,
> >
> >> diff --git a/drivers/iommu/intel-iommu-debugfs.c
> >> b/drivers/iommu/intel- iommu-debugfs.c index
> >> 73a552914455..e31c3b416351 100644
> >> --- a/drivers/iommu/intel-iommu-debugfs.c
> >> +++ b/drivers/iommu/intel-iommu-debugfs.c
> >> @@ -235,7 +235,7 @@ static void ctx_tbl_walk(struct seq_file *m,
> >> struct intel_iommu *iommu, u16 bus)
> >>   		tbl_wlk.ctx_entry = context;
> >>   		m->private = &tbl_wlk;
> >>
> >> -		if (pasid_supported(iommu) && is_pasid_enabled(context)) {
> >> +		if (dmar_readq(iommu->reg + DMAR_RTADDR_REG) &
> >> DMA_RTADDR_SMT) {
> >
> > Thanks for adding this, I do believe this is a good addition but I
> > also think that we might need "is_pasid_enabled()" as well. It checks if PASIDE
> bit in context entry is enabled or not.
> >
> > I am thinking that even though DMAR might be using scalable root and
> > context table, the entry itself should have PASIDE bit set. Did I miss something
> here?
> 
> No matter the PASIDE bit set or not, IOMMU always uses the scalable mode
> page table if scalable mode is enabled. If PASIDE is set, requests with PASID will
> be handled. Otherwise, requests with PASID will be blocked (but request without
> PASID will always be handled).
> 
> We are dumpling the page table of the IOMMU, so we only care about what
> page table format it is using. Do I understand it right>

Thanks! Baolu, for the explanation. Yes, it makes sense and I agree that we don't need the extra check for PASIDE bit.

I have tested this change on scalable/legacy DMAR's with/without iommu=pt and it works :)

Regards,
Sai