Re: [PATCH v2 09/17] xen/dom0less: store xenstore event channel in page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.07.2025 08:19, Juergen Gross wrote:
> On 16.07.25 23:14, Jason Andryuk wrote:
>> Write the associated event channel into the xenstore page so xenstored
>> can read it.  xenstored can map the grant by the reserved grant table
>> entry, and then read out the event channel and bind it.  This eliminates
>> the need for an additional mechanism to discover the event channel.
>>
>> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
>> ---
>>   xen/common/device-tree/dom0less-build.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/xen/common/device-tree/dom0less-build.c 
>> b/xen/common/device-tree/dom0less-build.c
>> index a2789e3bdf..284139bf14 100644
>> --- a/xen/common/device-tree/dom0less-build.c
>> +++ b/xen/common/device-tree/dom0less-build.c
>> @@ -26,6 +26,7 @@
>>   #include <public/io/xs_wire.h>
>>   
>>   #include <asm/dom0less-build.h>
>> +#include <asm/guest_access.h>
>>   #include <asm/setup.h>
>>   
>>   #include <xen/static-memory.h>
>> @@ -126,8 +127,14 @@ 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 */);
> 
> I couldn't find access_guest_memory_by_gpa() for x86. I think you either need 
> to
> introduce it for all archs, or you need to handle this differently.

This code isn't buildable (yet) for x86, is it? There's a series of Alejandro's
moving in that direction, but that didn't go in yet. And even then aiui this
file would still be excluded.

Jan