Re: [PATCH v1 5/6] xen: move domain_use_host_layout() to common header

[Jan Beulich <jbeulich@suse.com>]

On 18.02.2026 13:58, Oleksii Kurochko wrote:
> 
> On 2/17/26 8:34 AM, Jan Beulich wrote:
>> On 16.02.2026 19:42, Stefano Stabellini wrote:
>>> On Mon, 16 Feb 2026, Jan Beulich wrote:
>>>> On 12.02.2026 17:21, Oleksii Kurochko wrote:
>>>>> domain_use_host_layout() is generic enough to be moved to the
>>>>> common header xen/domain.h.
>>>> Maybe, but then something DT-specific, not xen/domain.h. Specifically, ...
>>>>
>>>>> --- a/xen/include/xen/domain.h
>>>>> +++ b/xen/include/xen/domain.h
>>>>> @@ -62,6 +62,22 @@ void domid_free(domid_t domid);
>>>>>   #define is_domain_direct_mapped(d) ((d)->cdf & CDF_directmap)
>>>>>   #define is_domain_using_staticmem(d) ((d)->cdf & CDF_staticmem)
>>>>>   
>>>>> +/*
>>>>> + * Is the domain using the host memory layout?
>>>>> + *
>>>>> + * Direct-mapped domain will always have the RAM mapped with GFN == MFN.
>>>>> + * To avoid any trouble finding space, it is easier to force using the
>>>>> + * host memory layout.
>>>>> + *
>>>>> + * The hardware domain will use the host layout regardless of
>>>>> + * direct-mapped because some OS may rely on a specific address ranges
>>>>> + * for the devices.
>>>>> + */
>>>>> +#ifndef domain_use_host_layout
>>>>> +# define domain_use_host_layout(d) (is_domain_direct_mapped(d) || \
>>>>> +                                    is_hardware_domain(d))
>>>> ... is_domain_direct_mapped() isn't something that I'd like to see further
>>>> proliferate in common (non-DT) code.
>>> Hi Jan, we have a requirement for 1:1 mapped Dom0 (I should say hardware
>>> domain) on x86 as well. In fact, we already have a working prototype,
>>> although it is not suitable for upstream yet.
>>>
>>> In addition to the PSP use case that we discussed a few months ago,
>>> where the PSP is not behind an IOMMU and therefore exchanged addresses
>>> must be 1:1 mapped, we also have a new use case. We are running the full
>>> Xen-based automotive stack on an Azure instance where SVM (vmentry and
>>> vmexit) is available, but an IOMMU is not present. All virtual machines
>>> are configured as PVH.
>> Hmm. Then adjustments need making, for commentary and macro to be correct
>> on x86. First and foremost none of what is there is true for PV.
> 
> As is_domain_direct_mapped() returns always false for x86, so
> domain_use_host_layout macro will return incorrect value for non-hardware
> domains (dom0?). And as PV domains are not auto_translated domains so are
> always direct-mapped, so technically is_domain_direct_mapped() (or
> domain_use_host_layout()) should return true in such case.

Hmm? PV domains aren't direct-mapped. Direct-map was introduced by Arm for
some special purpose (absence of IOMMU iirc).

> (I assume it is also true for every domain except HVM according to the comment
> /* HVM guests are translated.  PV guests are not. */ in xc_dom_translated and
> the comment above definition of XENFEAT_direct_mapped: /* ...not auto_translated
> domains (x86 only) are always direct-mapped*/).
> 
> Is my understanding correct?
> 
> Then isn't that a problem of how is_domain_direct_mapped() is defined
> for x86? Shouldn't it be defined like:
>    #define is_domain_direct_mapped(d) (!paging_mode_translate(d) || ((d)->cdf & CDF_directmap))
> 
> Would it be better to move "!paging_mode_translate(d) || " to the definition
> of domain_use_host_layout()?
> 
> Could you please explain what is wrong with the comment? Probably, except:
>    * To avoid any trouble finding space, it is easier to force using the
>    * host memory layout.
> everything else should be true for x86.

"The hardware domain will use ..." isn't true for PV Dom0.

Jan