Re: [PATCH] xen/iommu: arm: Don't insert an IOMMU mapping when the grantee and granter...

From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
Cc: Julien Grall <jgrall@amazon.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	Rahul Singh <Rahul.Singh@arm.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH] xen/iommu: arm: Don't insert an IOMMU mapping when the grantee and granter...
Date: Mon, 15 Feb 2021 09:13:14 +0100	[thread overview]
Message-ID: <3a827099-1d8f-826d-42ef-743d86d9ccce@suse.com> (raw)
In-Reply-To: <20210214143504.23099-1-julien@xen.org>

On 14.02.2021 15:35, Julien Grall wrote:
> From: Julien Grall <jgrall@amazon.com>
> 
> ... are the same.
> 
> When the IOMMU is enabled and the domain is direct mapped (e.g. Dom0),
> Xen will insert a 1:1 mapping for each grant mapping in the P2M to
> allow DMA.
> 
> This works quite well when the grantee and granter and not the same
> because the GFN in the P2M should not be mapped. However, if they are
> the same, we will overwrite the mapping. Worse, it will be completely
> removed when the grant is unmapped.
> 
> As the domain is direct mapped, a 1:1 mapping should always present in
> the P2M. This is not 100% guaranteed if the domain decides to mess with
> the P2M. However, such domain would already end up in trouble as the
> page would be soon be freed (when the last reference dropped).
> 
> Add an additional check in arm_iommu_{,un}map_page() to check whether
> the page belongs to the domain. If it is belongs to it, then ignore the
> request.

Doesn't this want / need solving in grant_table.c itself, as it also
affects PV on x86? Or alternatively in gnttab_need_iommu_mapping(),
handing the macro the MFN alongside the domain? No matter which one
was chosen, it could at the same time avoid the expensive mapkind()
invocation in this case.

Jan