Re: [PATCH 1/6] xen: extend XEN_DOMCTL_memory_mapping to handle cacheability

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

Hi,

On 4/17/19 10:12 PM, Stefano Stabellini wrote:
> On Wed, 27 Feb 2019, Jan Beulich wrote:
>>>>> On 27.02.19 at 00:07, <sstabellini@kernel.org> wrote:
>>> --- a/xen/include/public/domctl.h
>>> +++ b/xen/include/public/domctl.h
>>> @@ -571,12 +571,14 @@ struct xen_domctl_bind_pt_irq {
>>>   */
>>>   #define DPCI_ADD_MAPPING         1
>>>   #define DPCI_REMOVE_MAPPING      0
>>> +#define CACHEABILITY_DEVMEM      0 /* device memory, the default */
>>> +#define CACHEABILITY_MEMORY      1 /* normal memory */
>>>   struct xen_domctl_memory_mapping {
>>>       uint64_aligned_t first_gfn; /* first page (hvm guest phys page) in range */
>>>       uint64_aligned_t first_mfn; /* first page (machine page) in range */
>>>       uint64_aligned_t nr_mfns;   /* number of pages in range (>0) */
>>>       uint32_t add_mapping;       /* add or remove mapping */
>>> -    uint32_t padding;           /* padding for 64-bit aligned structure */
>>> +    uint32_t cache_policy;      /* cacheability of the memory mapping */
>>>   };
>>
>> I don't think DEVMEM and MEMORY are anywhere near descriptive
>> enough, nor - if we want such control anyway - flexible enough. I
>> think what you want is to actually specify cachability, allowing on
>> x86 to e.g. map frame buffers or alike WC. The attribute then
>> would (obviously and necessarily) be architecture specific.
> 
> Yes, I agree with what you wrote, and also with what Julien wrote. Now
> the question is how do you both think this should look like in more
> details:
> 
> - are you OK with using memory_policy instead of cache_policy like
>    Julien's suggested as name for the field?
> - are you OK with using #defines for the values?
> - should the #defines for both x86 and Arm be defined here or in other
>    headers?
> - what values would you like to see for x86?
> 
> For Arm, the ones I care about are:
> 
> - p2m_mmio_direct_dev
> - p2m_mmio_direct_c

First step first. What are the reason to have p2m_mmio_direct_c? Is it 
only for memory sharing between DomU and Dom0?

Cheers,

-- 
Julien Grall

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