linux-kernel - Re: [PATCH] iommu/vt-d: fix iommu pasid memory alloc & max pasid err

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Message-ID: <20250902162859.GE184112@ziepe.ca>
Date: Tue, 2 Sep 2025 13:28:59 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Guanghui Feng <guanghuifeng@...ux.alibaba.com>, dwmw2@...radead.org,
	joro@...tes.org, will@...nel.org, robin.murphy@....com,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	alikernel-developer@...ux.alibaba.com
Subject: Re: [PATCH] iommu/vt-d: fix iommu pasid memory alloc & max pasid err

On Mon, Sep 01, 2025 at 03:28:29PM +0800, Baolu Lu wrote:
> On 8/30/25 21:07, Guanghui Feng wrote:
> > When intel_pasid_alloc_table allocates memory for Scalable Mode PASID
> > directories, the specified memory page order is incorrect, and an
> > additional PAGE_SHIFT is added. There is also an error in calculating
> > the maximum number of supported PASID directories. In the revised
> > implementation, 1 << (order + PASID_PDE_SHIFT - 3) represents the memory
> > occupied by the Scalable Mode PASID directory, divided by 8 to represent
> > the number of PASID directories, and then multiplied by the number of (1
> > << PASID_PDE_SHIFT) entries in each PASID directory.
> 
> Do you see any specific issues if the changes described in this patch
> are lacking?
> 
> > 
> > Signed-off-by: Guanghui Feng <guanghuifeng@...ux.alibaba.com>
> > ---
> >   drivers/iommu/intel/pasid.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
> > index 52f678975da7..9969913b600b 100644
> > --- a/drivers/iommu/intel/pasid.c
> > +++ b/drivers/iommu/intel/pasid.c
> > @@ -61,14 +61,14 @@ int intel_pasid_alloc_table(struct device *dev)
> >   	size = max_pasid >> (PASID_PDE_SHIFT - 3);
> >   	order = size ? get_order(size) : 0;
> >   	dir = iommu_alloc_pages_node_sz(info->iommu->node, GFP_KERNEL,
> > -					1 << (order + PAGE_SHIFT));
> > +					1 << order);
> 
> This converts the order to the allocation size.

Yeah, I don't understand this patch at all, 

iommu_alloc_pages_node_sz() takes bytes and get_order(size) returns
order.

  bytes == 1 << (get_order(size) + PAGE_SHIFT)

Is correct

So why is this being changed?

Jason