linux-kernel - Re: [RFC PATCH 4/7] iommu/amd: Initial support for AMD IOMMU v2 page table

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Date:   Thu, 18 Mar 2021 16:28:10 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Jon.Grimm@....com, Wei.Huang2@....com
Subject: Re: [RFC PATCH 4/7] iommu/amd: Initial support for AMD IOMMU v2 page
 table

Hi Suravee,

On Fri, Mar 12, 2021 at 03:04:08AM -0600, Suravee Suthikulpanit wrote:
> @@ -503,6 +504,7 @@ struct amd_io_pgtable {
>  	int			mode;
>  	u64			*root;
>  	atomic64_t		pt_root;    /* pgtable root and pgtable mode */
> +	struct mm_struct	v2_mm;
>  };

A whole mm_struct is a bit too much when all we really need is an 8-byte
page-table root pointer.


> +static pte_t *fetch_pte(struct amd_io_pgtable *pgtable,
> +		      unsigned long iova,
> +		      unsigned long *page_size)
> +{
> +	int level;
> +	pte_t *ptep;
> +
> +	ptep = lookup_address_in_mm(&pgtable->v2_mm, iova, &level);
> +	if (!ptep || pte_none(*ptep) || (level == PG_LEVEL_NONE))
> +		return NULL;

So you are re-using the in-kernel page-table building code. That safes
some lines of code, but has several problems:

	1) When you boot a kernel with this code on a machine with
	   5-level paging, the IOMMU code will build a 5-level
	   page-table too, breaking IOMMU mappings.

	2) You need a whole mm_struct per domain, which is big.

	3) The existing macros for CPU page-tables require locking. For
	   IOMMU page-tables this is not really necessary and might
	   cause scalability issues.

Overall I think you should write your own code to build a 4-level
page-table and use cmpxchg64 to avoid the need for locking. Then things
will not break when such a kernel is suddenly booted on a machine which
as 5-level paging enabled.

Regards,

	Joerg