Re: [PATCH for-4.9] xen/arm: acpi: Map MMIO on fault in stage-2 page table for the hardware domain

[Shanker Donthineni <shankerd@codeaurora.org>]

Hi Julien,


On 03/30/2017 07:43 AM, Julien Grall wrote:
> When booting using ACPI, not all MMIOs can be discovered by parsing the
> static tables or the UEFI memory map. A lot of them will be described in
> the DSDT. However, Xen does not have an AML parser which requires us to
> find a different approach.
>
> During the first discussions on supporting ACPI (see design doc [1]), it
> was decided to rely on the hardware domain to make a request to the
> hypervisor to map the MMIO region in stage-2 page table before accessing
> it. This approach works fine if the OS has limited hooks to modify the
> page tables.
>
> In the case of Linux kernel, notifiers have been added to map
> the MMIO regions when adding a new AMBA/platform device. Whilst this is
> covering most of the MMIOs, some of them (e.g OpRegion, ECAM...) are not
> related to a specific device or the driver is not using the
> AMBA/platform API. So more hooks would need to be added in the code.
>
> Various approaches have been discussed (see [2]), one of them was to
> create stage-2 mappings seamlessly in Xen upon hardware memory faults.
> This approach was first ruled out because it relies on the hardware
> domain to probe the region before any use. So this would not work when
> DMA'ing to another device's MMIO region when the device is protected by
> an SMMU. It has been pointed out that this is a limited use case compare
> to DMA'ing between MMIO and RAM.
>
> This patch implements this approach. All MMIOs region will be mapped in
> stage-2 using p2m_mmio_direct_c (i.e normal memory outer and inner
> write-back cacheable). The stage-1 page table will be in control of the
> memory attribute. This is fine because the hardware domain is a trusted
> domain.
>
> Note that MMIO will only be mapped on a data abort fault. It is assumed
> that it will not be possible to execute code from MMIO
> (p2m_mmio_direct_c will forbid that).
>
> As mentioned above, this solution will cover most of the cases. If a
> platform requires to do DMA'ing to another device's MMIO region without
> any access performed by the OS. Then it will be expected to have
> specific platform code in the hypervisor to map the MMIO at boot time or
> the OS to use the existing hypercalls (i.e XENMEM_add_to_add_physmap{,_batch})
> before any access.
>
> [1] https://lists.xen.org/archives/html/xen-devel/2015-11/msg00488.html
> [2] https://marc.info/?l=linux-arm-kernel&m=148469169210500&w=2
>
> Signed-off-by: Julien Grall <julien.grall@arm.com>
>
> ---
>     This patch is a candidate for Xen 4.9. Whilst the last posting date
>     was few days ago, I think it would be good to have this patch in the
>     next release. Although ACPI support for ARM64 is still a technical
>     preview, it would help servers to test Xen upstream on their platform
>     with ACPI and provide feedback on missing features.
>
>     All the code is gated by !acpi_disabled and therefore will not be
>     executed on when using Device Tree.
>
>     RM hat on: I will leave the decision to Stefano.
>
>     Shanker: You mentioned offline that you tested the patch. May I add
>     your tested-by?

Sure add my tested-by. If you want I can test one more time this mailing list patch.

-- 
Shanker Donthineni
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.


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