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

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Message-ID: <aMxUe7WLzMXJY16c@skinsburskii.localdomain>
Date: Thu, 18 Sep 2025 11:50:35 -0700
From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
To: Nuno Das Neves <nunodasneves@...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 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.

> +
> +	memset(&mshv_root.vmm_caps, 0, sizeof(mshv_root.vmm_caps));

Zeroying is redundant as mshv_root is a statci variable.

> +	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?

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
>