linux-kernel - Re: [PATCH next] iommu/vt-d: Use shallowest supported table depth in sagaw

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Message-ID: <20251128000448.GD738503@ziepe.ca>
Date: Thu, 27 Nov 2025 20:04:48 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Calvin Owens <calvin@...nvd.org>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	Lu Baolu <baolu.lu@...ux.intel.com>,
	Nicolin Chen <nicolinc@...dia.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH next] iommu/vt-d: Use shallowest supported table depth in
 sagaw

On Thu, Nov 27, 2025 at 12:39:39PM -0800, Calvin Owens wrote:
> On Thursday 11/27 at 14:32 -0400, Jason Gunthorpe wrote:
> > On Thu, Nov 27, 2025 at 09:06:35AM -0800, Calvin Owens wrote:
> > > The failing condition in paging_domain_compatible_second_stage() is:
> > > 
> > >     /* Page table level is supported. */
> > >     if (!(cap_sagaw(iommu->cap) & BIT(pt_info.aw)))
> > >         return -EINVAL;
> > > 
> > > This happens because, for many domains on this machine, MGAW=39 but
> > > SAGAW=0x04: that claims a 39-bit maximum address width, but also claims
> > > to only support 48-bit/4-level paging, which seems odd.
> > 
> > This logic was intended to deal with this:
> > 
> > static int compute_vasz_lg2_ss(struct intel_iommu *iommu)
> > {
> > 	unsigned int sagaw = cap_sagaw(iommu->cap);
> > 	unsigned int mgaw = cap_mgaw(iommu->cap);
> > 
> > 	/*
> > 	 * Find the largest table size that both the mgaw and sagaw support.
> > 	 * This sets both the number of table levels and the valid range of
> > 	 * IOVA.
> > 	 */
> > 	if (mgaw >= 48 && (sagaw & BIT(3)))
> > 		return min(57, mgaw);
> > 	else if (mgaw >= 39 && (sagaw & BIT(2)))
> > 		return min(48, mgaw);
> > 	else if (mgaw >= 30 && (sagaw & BIT(1)))
> > 		return min(39, mgaw);
> > 
> > Though it is not right..
> > 
> > I didn't consider that this would come up, the only way to solve it is
> > to pass in more information so the tree can be higher than the vasz
> > requires..
> > 
> > Can you try these two commits?
> > 
> > https://github.com/jgunthorpe/linux/commits/iommu_pt_vtd/
> 
> That works on the Skylake machine. If you want it:
> 
> Tested-by: Calvin Owens <calvin@...nvd.org>

Thanks!!

Jason