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

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

On 2025-08-27 04:03, Orzel, Michal wrote:
> 
> 
> On 26/08/2025 23:08, 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@amd.com>
>> ---
>> v2:
>> No change
>>
>> This should go in after the init-dom0less changes so init-dom0less is
>> ready for xenstored automatically introducing domains.
>>
>> I'm looking for feedback.  This is ARM-only for the time being, but that
>> is the only in-tree user of this code.  From the perspective, it is okay
>> to go in.
>>
>> If we want a cross-arch approach, a common function to write to guest
>> physical addresses would be needed for additional arches, but they
>> aren't available yet.
>>
>> Oleksii added a function pointer to dtb_load() and initrd_load() when
>> moving dom0less to common, but I think that isn't necessary.  Just
>> having a common helper would be sufficient.
>>
>> copy_to_guest_phys() or something_copy_to_guest_phys() could be defined
>> or a wrapper for ARM's copy_to_guest_phys_flush_dcache().  Other arches
>> could need to implement it when using dom0less.
>>
>> I'm not an ARM expert, but Stefano said
>> copy_to_guest_phys_flush_dcache() is not necessary since this xenstore
>> page isn't expected to be accessed without caches enabled.
> I'm not sure I understand this point. When copying data *to* the guest, cleaning
> is about Xen's cache, not guest's...

I was trying to highlight that the patch is using 
access_guest_memory_by_gpa(), but dtb_load() and initrd_load() use 
copy_to_guest_phys_flush_dcache().

I assumed from the name, and Stefano's comment, that 
copy_to_guest_phys_flush_dcache() was about ensuring the CPU's cache is 
flushed to RAM.  That way a guest starting with cache disabled would see 
the correct contents.  But I don't really know how it works and may be 
wrong.

Thanks,
Jason