Re: [PATCH v3 8/8] xen/dom0less: store xenstore event channel in page

[Jason Andryuk <jason.andryuk@amd.com>]

On 2025-08-27 03:58, Jan Beulich wrote:
> On 26.08.2025 23:08, Jason Andryuk wrote:
>> --- a/xen/common/device-tree/dom0less-build.c
>> +++ b/xen/common/device-tree/dom0less-build.c
>> @@ -26,6 +26,7 @@
>>   #include <public/event_channel.h>
>>   #include <public/io/xs_wire.h>
>>   
>> +#include <asm/guest_access.h>
>>   #include <asm/setup.h>
>>   
>>   #include <xen/static-memory.h>
>> @@ -120,8 +121,14 @@ static void __init initialize_domU_xenstore(void)
>>   
>>           if ( gfn != XENSTORE_PFN_LATE_ALLOC && IS_ENABLED(CONFIG_GRANT_TABLE) )
>>           {
>> +            evtchn_port_t port = d->arch.hvm.params[HVM_PARAM_STORE_EVTCHN];
>> +            paddr_t evtchn_gaddr = gfn_to_gaddr(_gfn(gfn)) +
>> +                offsetof(struct xenstore_domain_interface, evtchn_port);
>> +
>>               ASSERT(gfn < UINT32_MAX);
>>               gnttab_seed_entry(d, GNTTAB_RESERVED_XENSTORE, xs_domid, gfn);
>> +            access_guest_memory_by_gpa(d, evtchn_gaddr, &port, sizeof(port),
>> +                                       true /* is_write */);
> 
> Isn't the use of an arch-specific function going to pose yet another issue
> for making this code usable on x86? Can't you use copy_to_guest_phys() here?
> Which may in turn need to be passed in by the caller, see e.g. dtb_load()
> and initrd_load() (i.e. cache flushing may also be necessary for Arm).

Yes, that could be done, but it's not my preferred approach.  Using a 
function pointer to pass a compile time constant seems to me like a 
misuse of a function pointer.

I'd rather each arch using dom0less define:
unsigned long copy_to_guest_phys(struct domain *d,
                                  paddr_t gpa,
                                  void *buf,
                                  unsigned int len);

Which does the correct thing for the arch.

Alejandro was able to re-work things to re-use the dom0less parsing code 
(dom0less-bindings.c), but he has so far kept the x86 domain 
construction separate such that it does not use dom0less-build.c.  So I 
don't know how that will shake out.

But, yeah, I can just pass in a function pointer if that is what is 
agreed upon.

Regards,
Jason