Re: [Xen-devel] [PATCH V1] xen/arm: domain_build: introduce dom0_lowmem bootargs

[Peng Fan <van.freenix@xxxxxxxxx>]

Hi Edgar,
On Wed, Sep 14, 2016 at 04:16:58PM +0200, Edgar E. Iglesias wrote:
>On Wed, Sep 14, 2016 at 08:40:09PM +0800, Peng Fan wrote:
>> On Wed, Sep 14, 2016 at 01:34:10PM +0100, Julien Grall wrote:
>> >
>> >
>> >On 14/09/16 13:18, Peng Fan wrote:
>> >>Hello Julien,
>> >>
>> >>On Wed, Sep 14, 2016 at 01:06:01PM +0100, Julien Grall wrote:
>> >>>
>> >>>
>> >>>On 14/09/16 13:03, Peng Fan wrote:
>> >>>>Hello Julien,
>> >>>
>> >>>Hello Peng,
>> >>>
>> >>>>On Wed, Sep 14, 2016 at 11:47:10AM +0100, Julien Grall wrote:
>> >>>>>Hello,
>> >>>>>
>> >>>>>On 14/09/16 08:41, Peng Fan wrote:
>> >>>>>>On Wed, Sep 14, 2016 at 08:23:24AM +0100, Julien Grall wrote:
>> >>>>>>diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
>> >>>>>>index 35ab08d..cc71e6f 100644
>> >>>>>>--- a/xen/arch/arm/domain_build.c
>> >>>>>>+++ b/xen/arch/arm/domain_build.c
>> >>>>>>@@ -28,6 +28,8 @@
>> >>>>>>
>> >>>>>>static unsigned int __initdata opt_dom0_max_vcpus;
>> >>>>>>integer_param("dom0_max_vcpus", opt_dom0_max_vcpus);
>> >>>>>>+static bool_t __initdata opt_dom0_use_lowmem;
>> >>>>>>+boolean_param("dom0_use_lowmem", opt_dom0_use_lowmem);
>> >>>>>>
>> >>>>>>int dom0_11_mapping = 1;
>> >>>>>>
>> >>>>>>@@ -244,7 +246,7 @@ static void allocate_memory(struct domain *d, 
>> >>>>>>struct
>> >>>>>>kernel_info *kinfo)
>> >>>>>>   unsigned int order = get_11_allocation_size(kinfo->unassigned_mem);
>> >>>>>>   int i;
>> >>>>>>       
>> >>>>>>-    bool_t lowmem = is_32bit_domain(d);
>> >>>>>>+    bool_t lowmem = is_32bit_domain(d) || opt_dom0_use_lowmem;
>> >>>>>>   unsigned int bits;
>> >>>>>>
>> >>>>>>
>> >>>>>>Pass "dom0_use_lowmem=1" to xen to allocate lowmem as much as possible.
>> >>>>>
>> >>>>>Again, what is the benefit to have a command line option for that?
>> >>>>
>> >>>>Then you prefer directly change "bool_t lowmem = is_32bit_domain(d);" to 
>> >>>>"bool_t lowmem = true" ?
>> >>>>I just want to give user a choice.
>> >>>
>> >>>We don't add new command line parameter just because they look cool to 
>> >>>have.
>> >>>So far, you did not explain why it would be good to let the choice to the
>> >>>user and how it could be used.
>> >>
>> >>I have not try, if there is no lowmem.
>> >>
>> >>I have not look into alloc_domheap_pages.
>> >>I am not sure whether there is such a platform or not,
>> >>just thinking if there is soc that dram memory starts from 4GB, and there 
>> >>is no dram
>> >>below 4GB. If we still can get memory when lowmem is true, I am ok to 
>> >>change directly assign
>> >>lowmem with value true. Anyway I have not look into the internals of 
>> >>domheap and
>> >>not sure whether there is such a platform that no lowmem (:-
>> >
>> >We cannot exclude this possibility. However, the only reason that Xen is
>> >requiring to allocate a bank below 4GB for 32-bit domain is to handle
>> >non-LPAE kernel.
>> 
>> Now also need to handle device that have DMA limitation -:)
>
>Hi Peng,
>
>Doesn't your platform have an IOMMU/SMMU?

We have SMMU. This is not related to SMMU. Dom0 use 1:1 mapping and no SMMU 
involved,
the physical memory assigned to Dom0 maybe higher than 4GB, but Some IPs only
supports 32bits DMA in Dom0. Then assign a 64bits dma address to a device only 
supports 32
bits device in Linux will hang the device or else.

Regards,
Peng.
>
>Cheers,
>Edgar

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