Re: [PATCH V4 5/5] x86/hyperv: Implement root partition IOMMU kernel only driver

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] iommu/hyperv: missing !hv_root_partition() check in hv_root_iommu_detect()
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commit c89a0e9d0fe34e767bd4527e0d99839504bcbea1
Author: Mukesh R <mrathor@linux.microsoft.com>

x86/hyperv: Implement root partition IOMMU kernel only driver

This commit implements a kernel-only virtual IOMMU driver for baremetal
Microsoft Hyper-V root partitions, handling paging domain management, device
attachment, and IOVA page mapping via hypercalls.

> diff --git a/drivers/iommu/hyperv/hv-iommu-root.c b/drivers/iommu/hyperv/hv-iommu-root.c
> new file mode 100644
> index 0000000000000..e0e3e9d4385e2
> --- /dev/null
> +++ b/drivers/iommu/hyperv/hv-iommu-root.c

[ ... ]

> +static int hv_iommu_get_caps(struct hv_output_get_iommu_capabilities *caps)
> +{
> +	u64 status;
> +	unsigned long flags;
> +	struct hv_input_get_iommu_capabilities *input;
> +	struct hv_output_get_iommu_capabilities *output;
> +
> +	local_irq_save(flags);
> +
> +	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
> +	output = *this_cpu_ptr(hyperv_pcpu_output_arg);
> +	memset(input, 0, sizeof(*input));
> +	input->partition_id = HV_PARTITION_ID_SELF;
> +	status = hv_do_hypercall(HVCALL_GET_IOMMU_CAPABILITIES, input, output);
> +	*caps = *output;
> +
> +	local_irq_restore(flags);
> +
> +	if (!hv_result_success(status))
> +		hv_status_err(status, "\n");
> +
> +	return hv_result_to_errno(status);
> +}
> +
> +static int __init hv_root_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);

[ ... ]

> +void __init hv_root_iommu_detect(void)
> +{
> +	if (no_iommu || iommu_detected || hv_l1vh_partition())
> +		return;
> +
> +	/* This is set for root partitions only, it is never set for non-root */
> +	if (!(ms_hyperv.misc_features & HV_DEVICE_DOMAIN_AVAILABLE))
> +		return;
> +
> +	iommu_detected = 1;
> +	x86_init.iommu.iommu_init = hv_root_iommu_init;
> +
> +	pci_request_acs();
> +}

[Severity: High]
As reported by Sashiko on v3 [1], should hv_root_iommu_detect() check
!hv_root_partition() instead of hv_l1vh_partition()?

During x86 boot, ms_hyperv_init_platform() calls hv_identify_partition_type()
in drivers/hv/hv_common.c, which forces hv_curr_partition_type to
HV_PARTITION_TYPE_GUEST whenever HV_ISOLATION is set so that Confidential VMs
do not execute root partition paths:

drivers/hv/hv_common.c:hv_identify_partition_type() {
	hv_curr_partition_type = HV_PARTITION_TYPE_GUEST;
	...
	if ((ms_hyperv.priv_high & HV_CREATE_PARTITIONS) &&
	    !(ms_hyperv.priv_high & HV_ISOLATION)) {
		...
	}
}

Later in boot, pci_iommu_alloc() calls hv_root_iommu_detect(). Because
hv_root_iommu_detect() only checks hv_l1vh_partition(), a guest partition
where HV_DEVICE_DOMAIN_AVAILABLE is set in ms_hyperv.misc_features will set
iommu_detected = 1 and x86_init.iommu.iommu_init = hv_root_iommu_init.

When hyperv_init() then calls hv_common_init() in drivers/hv/hv_common.c,
hyperv_pcpu_output_arg is only allocated if hv_output_page_exists() is true:

drivers/hv/hv_common.c:hv_common_init() {
	...
	if (hv_output_page_exists()) {
		hyperv_pcpu_output_arg = alloc_percpu(void *);
		BUG_ON(!hyperv_pcpu_output_arg);
	}
}

For HV_PARTITION_TYPE_GUEST (with CONFIG_MSHV_ROOT depending on
!HYPERV_VTL_MODE), hv_output_page_exists() is false and hyperv_pcpu_output_arg
remains NULL.

If CONFIG_HYPERV_PVIOMMU is disabled, pci_iommu_init() invokes
hv_root_iommu_init() -> hv_iommu_get_caps(), which dereferences
*this_cpu_ptr(hyperv_pcpu_output_arg) while hyperv_pcpu_output_arg is NULL. If
CONFIG_HYPERV_PVIOMMU is enabled, hyperv_init() overwrites
x86_init.iommu.iommu_init with hv_pviommu_init, which then fails with -ENODEV
because hv_root_iommu_detect() already set iommu_detected = 1.

Would checking !hv_root_partition() in hv_root_iommu_detect() avoid this by
excluding both L1VH and guest partitions?

[1] https://lore.kernel.org/r/20261103025150.3192119-6-mrathor@linux.microsoft.com

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Sashiko AI review · https://sashiko.dev/#/patchset/20261105020106.3496339-1-mrathor@linux.microsoft.com?part=5