linux-kernel - Re: [RFC v1 3/5] hyperv: Introduce new hypercall interfaces used by Hyper-V guest IOMMU

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Message-ID: <330d26ac-f1a2-4ee9-8cd7-20fd17db9f92@linux.microsoft.com>
Date: Fri, 9 Jan 2026 10:47:20 -0800
From: Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>
Cc: Yu Zhang <zhangyu1@...ux.microsoft.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
 easwar.hariharan@...ux.microsoft.com, "kys@...rosoft.com"
 <kys@...rosoft.com>, "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
 "wei.liu@...nel.org" <wei.liu@...nel.org>,
 "decui@...rosoft.com" <decui@...rosoft.com>,
 "lpieralisi@...nel.org" <lpieralisi@...nel.org>,
 "kwilczynski@...nel.org" <kwilczynski@...nel.org>,
 "mani@...nel.org" <mani@...nel.org>, "robh@...nel.org" <robh@...nel.org>,
 "bhelgaas@...gle.com" <bhelgaas@...gle.com>, "arnd@...db.de"
 <arnd@...db.de>, "joro@...tes.org" <joro@...tes.org>,
 "will@...nel.org" <will@...nel.org>,
 "robin.murphy@....com" <robin.murphy@....com>,
 "jacob.pan@...ux.microsoft.com" <jacob.pan@...ux.microsoft.com>,
 "nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>,
 "mrathor@...ux.microsoft.com" <mrathor@...ux.microsoft.com>,
 "peterz@...radead.org" <peterz@...radead.org>,
 "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [RFC v1 3/5] hyperv: Introduce new hypercall interfaces used by
 Hyper-V guest IOMMU

On 1/8/2026 10:47 AM, Michael Kelley wrote:
> From: Yu Zhang <zhangyu1@...ux.microsoft.com> Sent: Monday, December 8, 2025 9:11 PM
>>
>> From: Wei Liu <wei.liu@...nel.org>
>>

<snip>

>> +struct hv_input_get_iommu_capabilities {
>> +	u64 partition_id;
>> +	u64 reserved;
>> +} __packed;
>> +
>> +struct hv_output_get_iommu_capabilities {
>> +	u32 size;
>> +	u16 reserved;
>> +	u8  max_iova_width;
>> +	u8  max_pasid_width;
>> +
>> +#define HV_IOMMU_CAP_PRESENT (1ULL << 0)
>> +#define HV_IOMMU_CAP_S2 (1ULL << 1)
>> +#define HV_IOMMU_CAP_S1 (1ULL << 2)
>> +#define HV_IOMMU_CAP_S1_5LVL (1ULL << 3)
>> +#define HV_IOMMU_CAP_PASID (1ULL << 4)
>> +#define HV_IOMMU_CAP_ATS (1ULL << 5)
>> +#define HV_IOMMU_CAP_PRI (1ULL << 6)
>> +
>> +	u64 iommu_cap;
>> +	u64 pgsize_bitmap;
>> +} __packed;
>> +
>> +enum hv_logical_device_property_code {
>> +	HV_LOGICAL_DEVICE_PROPERTY_PVIOMMU = 10,
>> +};
>> +
>> +struct hv_input_get_logical_device_property {
>> +	u64 partition_id;
>> +	u64 logical_device_id;
>> +	enum hv_logical_device_property_code code;
> 
> Historically we've avoided "enum" types in structures that are part of
> the hypervisor ABI. Use u32 here?

<snip>
What has been the reasoning for that practice? Since the introduction of the
include/hyperv/ headers, we have generally wanted to import as directly as
possible the relevant definitions from the hypervisor code base. If there's
a strong reason, we could consider switching the enum for a u32 here
since, at least for the moment, there's only a single value being used.

Thanks,
Easwar (he/him)