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

["Jan Beulich" <JBeulich@suse.com>]

>>> On 17.04.19 at 23:12, <sstabellini@kernel.org> 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?

Yes - in fact either is fine to me.

> - are you OK with using #defines for the values?

Yes.

> - should the #defines for both x86 and Arm be defined here or in other
>   headers?

I'd say here, but I wouldn't object to placement in arch-
specific public headers.

> - what values would you like to see for x86?

Unless you intend to implement the function for x86, I'd
suggest not adding any x86 #define-s at all for now.

But I agree with Julien (in case this wasn't explicit enough from
my earlier replay) that it first needs to be clarified whether such
an interface is wanted in the first place.

Jan



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