Re: [PATCH V1 3/3] x86/hyperv: Implement root VM IOMMU kernel only driver

["Jörg Rödel" <joro@8bytes.org>]

On Thu, Sep 24, 2026 at 05:14:12PM -0700, Mukesh R wrote:
> So, I'm at a bit of loss. I could go back to V0 which did what what you
> suggested above, but it can also leave stale mappings when map hypercall
> succeeds partially and we delete the entire tree node in cleanup (but
> we could add to the tree back whatever succeeded, but that could fail
> also, but then we could unmap whatever was mapped, but that could fail
> also.. see :)...).
> 

The root-problem seems to be that every operation in the step can fail,
inluding the unmap operation. This makes it really hard to maintain a
consistent state. Are there any guarantees Hyper-V gives on the unmap
operation, e.g under which circumstances it can fail? It would really help if
the code could assume that it will never fail, or treat a failure as a hard
error.

If there are no guarantees HV can give, a solution might be to add a
mapping-resize operation to the internal tree (which can not fail) to correctly
update the tree structure on partially successful unmaps.

I still believe that the internal tree structure should be updated before the
HV map operation takes place.

> > > +static int __init hv_iommu_init(void)
> > > +{
> > > +	int rc;
> > > +	struct iommu_device *iommup = &hv_virt_iommu;
> > > +	struct hv_output_get_iommu_capabilities caps;
> > > +
> > > +	if (!hv_is_hyperv_initialized())
> > > +		return -ENODEV;
> > > +
> > > +	rc = hv_iommu_get_caps(&caps);
> > > +	if (rc)
> > > +		return rc;
> > > +
> > 
> > The capabilities returned need more checking. I think at least it needs a check
> > for HV_IOMMU_CAP_PRESENT and that PAGE_SIZE is set in the pgsize_bitmap.
> 
> In general this caps hypercall is not available to root partitions,
> our only agreement is for hyp to provide max_iova_width. But, we check
> HV_DEVICE_DOMAIN_AVAILABLE in hv_iommu_detect() which sorta supersedes
> HV_IOMMU_CAP_PRESENT. As for page size, hyp owns the iommu and can chose
> the page size, meaning it can automatically use larger page size when
> possible. But the hypercall HVCALL_MAP_DEVICE_GPA_PAGES will only take
> 4k pfns as input, hence:
> 
> #define HV_IOMMU_PGSIZES SZ_4K        /* for now, to be enhanced */

Okay, so this deserves a comment in the code to make it clear why there is no
additional checking and what the hyper-v to guest contract for feature
detection is.

> Ok, I can change it back to DMA_BIT_MASK. (I removed after comment
> on V0 that this is not dma address).

An IOVA is a DMA address by definition :)

-Joerg