linux-kernel - Re: [PATCH v0 12/15] x86/hyperv: Implement hyperv virtual iommu

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Message-ID: <ace4f4fd-f328-cfdc-00eb-6751c8c9ebb0@linux.microsoft.com>
Date: Fri, 30 Jan 2026 14:10:57 -0800
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>
Cc: linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
 linux-pci@...r.kernel.org, linux-arch@...r.kernel.org, kys@...rosoft.com,
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 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, joro@...tes.org,
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 robh@...nel.org, bhelgaas@...gle.com, arnd@...db.de,
 nunodasneves@...ux.microsoft.com, mhklinux@...look.com
Subject: Re: [PATCH v0 12/15] x86/hyperv: Implement hyperv virtual iommu

On 1/27/26 14:31, Jacob Pan wrote:
> Hi Mukesh,
> 
>>>>> +
>>>>> +	if (hv_l1vh_partition() && !hv_curr_thread_is_vmm() &&
>>>>> !hv_no_attdev) {
>>>>> +		pr_err("Hyper-V: l1vh iommu does not support
>>>>> host devices\n");
>>>> why is this an error if user input choose not to do direct
>>>> attach?
>>>
>>> Like the error message says: on l1vh, direct attaches of host
>>> devices (eg dpdk) is not supported. and l1vh only does direct
>>> attaches. IOW, no host devices on l1vh.
>>>    
>> This hv_no_attdev flag is really confusing to me, by default
>> hv_no_attdev is false, which allows direct attach. And you are saying
>> l1vh allows it.
>>
>> Why is this flag also controls host device attachment in l1vh? If you
>> can tell the difference between direct host device attach and other
>> direct attach, why don't you reject always reject host attach in l1vh?
> On second thought, if the hv_no_attdev knob is only meant to control
> host domain attach vs. direct attach, then it is irrelevant on L1VH.
> 
> Would it make more sense to rename this to something like
> hv_host_disable_direct_attach? That would better reflect its scope and
> allow it to be ignored under L1VH, and reduce the risk of users
> misinterpreting or misusing it.

It would, but it is kernel parameter and needs to be terse. It would
be documented properly tho, so we should be ok.

Thanks,
-Mukesh