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

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Message-ID: <20260127143119.00006d2f@linux.microsoft.com>
Date: Tue, 27 Jan 2026 14:31:19 -0800
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: Mukesh R <mrathor@...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,
 haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
 longli@...rosoft.com, catalin.marinas@....com, will@...nel.org,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, joro@...tes.org,
 lpieralisi@...nel.org, kwilczynski@...nel.org, mani@...nel.org,
 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

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.