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

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@xxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>

This patch is to resolve the issue mentioned in
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

I have rebase this patch based on 4.8-unstable.
latest commit:
commit a3fe74e4345e66ddb7aa514395260a5e5f8b0cdc
Author: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
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 

Please kindly comments whether introudcing "dom0_lowmem" is acceptable or not
to resolve the issue I met in 

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 
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.


Julien Grall

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