Re: [PATCH 4/6] mshv: Get the vmm capabilities offered by the hypervisor

[Anirudh Rayabharam <anirudh@anirudhrb.com>]

On Thu, Aug 28, 2025 at 05:43:48PM -0700, Nuno Das Neves wrote:
> From: Purna Pavan Chandra Aekkaladevi <paekkaladevi@linux.microsoft.com>
> 
> Some newer hypervisor APIs are gated by feature bits in the so-called
> "vmm capabilities" partition property. Store the capabilities on
> mshv_root module init, using HVCALL_GET_PARTITION_PROPERTY_EX.
> 
> Signed-off-by: Purna Pavan Chandra Aekkaladevi <paekkaladevi@linux.microsoft.com>
> Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
> ---
>  drivers/hv/mshv_root.h      |  1 +
>  drivers/hv/mshv_root_main.c | 28 ++++++++++++++++++++++++++++
>  2 files changed, 29 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 56ababab57ce..29f61ecc9771 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -2327,6 +2327,28 @@ static int __init mshv_root_partition_init(struct device *dev)
>  	return err;
>  }
>  
> +static int mshv_init_vmm_caps(struct device *dev)
> +{
> +	int ret;
> +
> +	memset(&mshv_root.vmm_caps, 0, sizeof(mshv_root.vmm_caps));
> +	ret = hv_call_get_partition_property_ex(HV_PARTITION_ID_SELF,
> +						HV_PARTITION_PROPERTY_VMM_CAPABILITIES,
> +						0, &mshv_root.vmm_caps,
> +						sizeof(mshv_root.vmm_caps));
> +
> +	/*
> +	 * HV_PARTITION_PROPERTY_VMM_CAPABILITIES is not supported in
> +	 * older hyperv. Ignore the -EIO error code.
> +	 */
> +	if (ret && ret != -EIO)
> +		return ret;
> +
> +	dev_dbg(dev, "vmm_caps=0x%llx\n", mshv_root.vmm_caps.as_uint64[0]);
> +
> +	return 0;
> +}
> +
>  static int __init mshv_parent_partition_init(void)
>  {
>  	int ret;
> @@ -2377,6 +2399,12 @@ static int __init mshv_parent_partition_init(void)
>  	if (ret)
>  		goto remove_cpu_state;
>  
> +	ret = mshv_init_vmm_caps(dev);
> +	if (ret) {
> +		dev_err(dev, "Failed to get VMM capabilities\n");
> +		goto exit_partition;

Should this really be treated as a failure here? We could still offer
/dev/mshv albeit with potentially limited capabilities.

Thanks,
Anirudh.

> +	}
> +
>  	ret = mshv_irqfd_wq_init();
>  	if (ret)
>  		goto exit_partition;
> -- 
> 2.34.1
>