linux-kernel - Re: [PATCH v3 3/5] mshv: Get the vmm capabilities offered by the hypervisor

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Message-ID: <8d6c6f95-7ccd-4471-a391-39a45f95af04@linux.microsoft.com>
Date: Fri, 19 Sep 2025 15:37:03 -0700
From: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
 prapal@...ux.microsoft.com, easwar.hariharan@...ux.microsoft.com,
 tiala@...rosoft.com, anirudh@...rudhrb.com,
 paekkaladevi@...ux.microsoft.com, kys@...rosoft.com, haiyangz@...rosoft.com,
 wei.liu@...nel.org, decui@...rosoft.com
Subject: Re: [PATCH v3 3/5] mshv: Get the vmm capabilities offered by the
 hypervisor

On 9/18/2025 11:50 AM, Stanislav Kinsburskii wrote:
> On Tue, Sep 16, 2025 at 04:44:20PM -0700, Nuno Das Neves wrote:
>> From: Purna Pavan Chandra Aekkaladevi <paekkaladevi@...ux.microsoft.com>
>>
>> Some hypervisor APIs are gated by feature bits in the
>> "vmm capabilities" partition property. Store the capabilities on
>> mshv_root module init, using HVCALL_GET_PARTITION_PROPERTY_EX.
>>
>> This is not supported on all hypervisors. In that case, just set the
>> capabilities to 0 and proceed as normal.
>>
>> Signed-off-by: Purna Pavan Chandra Aekkaladevi <paekkaladevi@...ux.microsoft.com>
>> Signed-off-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
>> Reviewed-by: Praveen K Paladugu <prapal@...ux.microsoft.com>
>> Reviewed-by: Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>
>> Reviewed-by: Tianyu Lan <tiala@...rosoft.com>
>> ---
>>  drivers/hv/mshv_root.h      |  1 +
>>  drivers/hv/mshv_root_main.c | 22 ++++++++++++++++++++++
>>  2 files changed, 23 insertions(+)
>>
>> diff --git a/drivers/hv/mshv_root.h b/drivers/hv/mshv_root.h
>> index 4aeb03bea6b6..0cb1e2589fe1 100644
>> --- a/drivers/hv/mshv_root.h
>> +++ b/drivers/hv/mshv_root.h
>> @@ -178,6 +178,7 @@ struct mshv_root {
>>  	struct hv_synic_pages __percpu *synic_pages;
>>  	spinlock_t pt_ht_lock;
>>  	DECLARE_HASHTABLE(pt_htable, MSHV_PARTITIONS_HASH_BITS);
>> +	struct hv_partition_property_vmm_capabilities vmm_caps;
>>  };
>>  
>>  /*
>> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
>> index 24df47726363..f7738cefbdf3 100644
>> --- a/drivers/hv/mshv_root_main.c
>> +++ b/drivers/hv/mshv_root_main.c
>> @@ -2201,6 +2201,26 @@ static int __init mshv_root_partition_init(struct device *dev)
>>  	return err;
>>  }
>>  
>> +static void mshv_init_vmm_caps(struct device *dev)
>> +{
>> +	int ret;
> 
> nit: this is void function so ret looks redundant.
> 

True, it's not needed.

>> +
>> +	memset(&mshv_root.vmm_caps, 0, sizeof(mshv_root.vmm_caps));
> 
> Zeroying is redundant as mshv_root is a statci variable.
> 

Good point.

>> +	ret = hv_call_get_partition_property_ex(HV_PARTITION_ID_SELF,
>> +						HV_PARTITION_PROPERTY_VMM_CAPABILITIES,
>> +						0, &mshv_root.vmm_caps,
> 
> Also, we align "slow" hypercalls by PAGE_SIZE. Why is it fine to not do
> it here?
> 

I guess you're referring to the output argument of the hypercall?

Check the previous patch to see how hv_call_get_partition_property_ex()
is implemented. It uses the per-cpu input/output args as normal, which
are HV_HYP_PAGE_SIZE in size, and page-aligned.

> Thanks,
> Stanislav
> 
>> +						sizeof(mshv_root.vmm_caps));
>> +
>> +	/*
>> +	 * HVCALL_GET_PARTITION_PROPERTY_EX or HV_PARTITION_PROPERTY_VMM_CAPABILITIES
>> +	 * may not be supported. Leave them as 0 in that case.
>> +	 */
>> +	if (ret)
>> +		dev_warn(dev, "Unable to get VMM capabilities\n");
>> +
>> +	dev_dbg(dev, "vmm_caps=0x%llx\n", mshv_root.vmm_caps.as_uint64[0]);
>> +}
>> +
>>  static int __init mshv_parent_partition_init(void)
>>  {
>>  	int ret;
>> @@ -2253,6 +2273,8 @@ static int __init mshv_parent_partition_init(void)
>>  	if (ret)
>>  		goto remove_cpu_state;
>>  
>> +	mshv_init_vmm_caps(dev);
>> +
>>  	ret = mshv_irqfd_wq_init();
>>  	if (ret)
>>  		goto exit_partition;
>> -- 
>> 2.34.1
>>