Re: [RFC] xen/arm: domain_build: introduce dom0_lowmem bootargs

[Julien Grall <julien.grall@arm.com>]

Hello Peng,

On 14/09/2016 02:31, Peng Fan wrote:
> On Tue, Sep 13, 2016 at 02:24:31PM +0100, Julien Grall wrote:
>>
>>
>> On 13/09/16 14:12, Peng Fan wrote:
>>> Hi Julien,
>>> On Tue, Sep 13, 2016 at 01:59:01PM +0100, Julien Grall wrote:
>>>> Hello Peng,
>>>>
>>>> On 13/09/16 13:55, Peng Fan wrote:
>>>>> On AArch64 SoCs, some IPs may only have the capability to access
>>>>> 32bits address space. The physical memory assigned for Dom0 maybe
>>>>> not in 4GB address space, then the IPs will not work properly.
>>>>>
>>>>> Introduce dom0_lowmem bootargs, user could pass "dom0_lowmem=xx"
>>>>> to xen. It means how much memory user would like to be allocated
>>>>> in lower 4GB memory space. If there is not enough memory for
>>>>> dom0_lowmem, higher memory will be allocated.
>>>>>
>>>>> Thinking such a memory layout on an AArch64 SoC:
>>>>> Region 0: 2GB(0x80000000 - 0xffffffff)
>>>>> Region 1: 4GB(0x880000000 - 0x97fffffff)
>>>>> If user would like to assign 2GB for Dom0 and 1GB of the 2GB memory
>>>>> in Region 0, user could pass "dom0=2048M dom0_lowmem=1024M" to xen.
>>>>>
>>>>> Signed-off-by: Peng Fan <peng.fan@nxp.com>
>>>>> Cc: Stefano Stabellini <sstabellini@kernel.org>
>>>>> Cc: Julien Grall <julien.grall@arm.com>
>>>>> ---
>>>>>
>>>>> This patch is to resolve the issue mentioned in
>>>>> https://lists.xen.org/archives/html/xen-devel/2016-09/msg00235.html
>>>>> This patch not tested on latest 4.8-unstable, I only tested similar
>>>>> patch on xen 4.7 on AArch64 platform.
>>>>
>>>> Please test any patch send upstream on 4.8-unstable. The code may have
>>>> changed.
>>>
>>> I have rebase this patch based on 4.8-unstable.
>>> latest commit:
>>> "
>>> commit a3fe74e4345e66ddb7aa514395260a5e5f8b0cdc
>>> Author: Tamas K Lengyel <tamas.lengyel@zentific.com>
>>> Date:   Mon Aug 1 11:59:14 2016 -0600
>>>
>>>    arm/vm_event: get/set registers
>>> "
>>>
>>> Since I have not rebased my platform patches to 4.8-unstable, so have not tested.
>>>
>>> Please kindly comments whether introudcing "dom0_lowmem" is acceptable or not
>>> to resolve the issue I met in https://lists.xen.org/archives/html/xen-devel/2016-09/msg00235.html.
>>>
>>> I'll rebase my platform patches and do some test.
>>>
>>>>
>>>>>
>>>>> The idea of patch is that user could specify the lowmem that user would
>>>>> like to use. I rethought the ideas in https://lists.xen.org/archives/html/xen-devel/2016-09/msg00487.html,
>>>>> but that is not good. lowmem is precise, it maybe used for some IPs that maybe
>>>>> passthrough to DomU, so we only allocate the needed memory for Dom0.
>>>>
>>>> Why? IPs passthrough to DomU will have to be protected by an SMMU so it does
>>>> not matter whether Dom0 is using all the lowmem or not.
>>>
>>> I just think there maybe some cases that some physical memory need to be
>>> reserved for DomU usage.
>>> SMMU is not a must when we passthrough a non DMA capable device to DomU.
>>> So I think an DRAM area can be encoded in dts, and passthrough to DomU.
>>
>> How do you make sure that DomU will program the device with the correct
>> address? A malicious DomU could decide to use a physical address belonging to
>> another DomU or even Xen and would be able to read/write on it.
>
> When I debug DomU, I use a uart port as an early console for DomU.
> I just marked the uart with xen,passthrough and status disabled in Dom0 dts,
> then in DomU dts, I create a uart node, and in xl cfg, I mapped the address
> space. I did not use SMMU here. Just mapped the uart physical address
> to DomU uart guest physical address. And DomU can access the uart for my
> debug usage.

That's a different use case which does not involve UART reading directly 
the memory. Hence, there is no obligation to allocate lowmem for DomU.

Regards,

-- 
Julien Grall

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