linux-kernel - Re: [PATCH] xenbus: Allow PVH dom0 a non-local xenstore

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Message-ID: <63d6ff3c-bed5-4a7f-bd3e-50b99b5a6f4b@amd.com>
Date: Tue, 6 May 2025 09:25:34 -0400
From: Jason Andryuk <jason.andryuk@....com>
To: Stefano Stabellini <sstabellini@...nel.org>
CC: Juergen Gross <jgross@...e.com>, Oleksandr Tyshchenko
	<oleksandr_tyshchenko@...m.com>, <xen-devel@...ts.xenproject.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xenbus: Allow PVH dom0 a non-local xenstore

On 2025-05-05 16:44, Stefano Stabellini wrote:
> On Sat, 3 May 2025, Jason Andryuk wrote:
>> Make xenbus_init() allow a non-local xenstore for a PVH dom0 - it is
>> currently forced to XS_LOCAL.  With Hyperlaunch booting dom0 and a
>> xenstore stubdom, dom0 can be handled as a regular XS_HVM following the
>> late init path.
>>
>> Drop the use of xen_initial_domain() and just check for the event
>> channel instead.  This matches the PV case where there is no check for
>> initial domain.
>>
>> Check the full 64bit HVM_PARAM_STORE_EVTCHN value to catch the off
>> chance that high bits are set for the 32bit event channel.
>>
>> Signed-off-by: Jason Andryuk <jason.andryuk@....com>
> 
> Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>

Thanks, Stefano.  But I'm wondering if this might break ARM enhanced 
no-xenstore.

> 
>> ---
>>   drivers/xen/xenbus/xenbus_probe.c | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
>> index 6d32ffb01136..7604f70ee108 100644
>> --- a/drivers/xen/xenbus/xenbus_probe.c
>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>> @@ -966,9 +966,15 @@ static int __init xenbus_init(void)
>>   	if (xen_pv_domain())
>>   		xen_store_domain_type = XS_PV;
>>   	if (xen_hvm_domain())
>> +	{
>>   		xen_store_domain_type = XS_HVM;

ARM would have everything set to XS_HVM...

>> -	if (xen_hvm_domain() && xen_initial_domain())
>> -		xen_store_domain_type = XS_LOCAL;

...and only dom0 set to XS_LOCAL.

>> +		err = hvm_get_parameter(HVM_PARAM_STORE_EVTCHN, &v);
>> +		if (err)
>> +			goto out_error;
>> +		xen_store_evtchn = (int)v;
>> +		if (!v)
>> +			xen_store_domain_type = XS_LOCAL;
>> +	}
>>   	if (xen_pv_domain() && !xen_start_info->store_evtchn)
>>   		xen_store_domain_type = XS_LOCAL;
>>   	if (xen_pv_domain() && xen_start_info->store_evtchn)
>> @@ -987,10 +993,6 @@ static int __init xenbus_init(void)
>>   		xen_store_interface = gfn_to_virt(xen_store_gfn);
>>   		break;
>>   	case XS_HVM:
>> -		err = hvm_get_parameter(HVM_PARAM_STORE_EVTCHN, &v);
>> -		if (err)
>> -			goto out_error;
>> -		xen_store_evtchn = (int)v;
>>   		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>>   		if (err)
>>   			goto out_error;
                 /*
                  * Uninitialized hvm_params are zero and return no error.
                  * Although it is theoretically possible to have
                  * HVM_PARAM_STORE_PFN set to zero on purpose, in 
reality it is
                  * not zero when valid. If zero, it means that Xenstore 
hasn't
                  * been properly initialized. Instead of attempting to 
map a
                  * wrong guest physical address return error.
                  *
                  * Also recognize all bits set as an 
invalid/uninitialized value.
                  */
                 if (!v) {
                         err = -ENOENT;
                         goto out_error;
                 }

IIUC, this !v check is for enhanced no-xenstore to end up in XS_UNKNOWN. 
  I'll have to re-work to handle that case.

Regards,
Jason


>> -- 
>> 2.34.1
>>