linux-kernel - RE: [PATCH v2 1/1] iommu/vt-d: Fix kdump kernels boot failure with scalable mode

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Date:   Mon, 22 Aug 2022 04:42:47 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
CC:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        "Jin, Wen" <wen.jin@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH v2 1/1] iommu/vt-d: Fix kdump kernels boot failure with
 scalable mode

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Thursday, August 18, 2022 7:13 PM
> >>   	rtaddr_reg = dmar_readq(iommu->reg + DMAR_RTADDR_REG);
> >> -	ext        = !!(rtaddr_reg & DMA_RTADDR_RTT);
> >> -	new_ext    = !!ecap_ecs(iommu->ecap);
> >> +	ext        = !!(rtaddr_reg & DMA_RTADDR_SMT);
> >> +	new_ext    = !!ecap_smts(iommu->ecap);
> >
> > should be !!sm_supported()
> 
> Not really. The IOMMU was setup by the previous kernel. Here we just
> check whether the scalable mode was enabled there.

You want to compare whether old kernel and new kernel enable
the same mode. ecap_smts is only about the capability. only 
sm_supported() can tell the mode which is actually used by the
new kernel.

> 
> >
> >>
> >>   	/*
> >>   	 * The RTT bit can only be changed when translation is disabled,
> >> @@ -2747,6 +2705,10 @@ static int copy_translation_tables(struct
> >> intel_iommu *iommu)
> >>   	if (new_ext != ext)
> >>   		return -EINVAL;
> >>
> >> +	iommu->copied_tables = bitmap_zalloc(BIT_ULL(16), GFP_KERNEL);
> >> +	if (!iommu->copied_tables)
> >> +		return -ENOMEM;
> >> +
> >>   	old_rt_phys = rtaddr_reg & VTD_PAGE_MASK;
> >>   	if (!old_rt_phys)
> >>   		return -EINVAL;
> >
> > Out of curiosity. What is the rationale that we copy root table and
> > context tables but not pasid tables?
> 
> We only copy the context table and reconstruct it when the default
> domain is attached. Before that, there's no need to reconstruct the
> pasid table, hence it's safe to use the previous pasid tables.
> 

I still didn't get why context table must be reconstructed but not
pasid table...