Message-ID: <4a7c63fc-b96a-9841-7745-adbc41190c36@linux.microsoft.com>
Date: Thu, 5 Feb 2026 09:57:20 -0800
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Stanislav Kinsburskii <skinsburskii@...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 15/15] mshv: Populate mmio mappings for PCI passthru

On 2/5/26 08:28, Stanislav Kinsburskii wrote:
> On Wed, Feb 04, 2026 at 02:52:54PM -0800, Mukesh R wrote:
>> On 2/2/26 08:30, Stanislav Kinsburskii wrote:
>>> On Fri, Jan 30, 2026 at 02:17:24PM -0800, Mukesh R wrote:
>>>> On 1/27/26 10:57, Stanislav Kinsburskii wrote:
>>>>> On Mon, Jan 26, 2026 at 07:07:22PM -0800, Mukesh R wrote:
>>>>>> On 1/26/26 10:15, Stanislav Kinsburskii wrote:
>>>>>>> On Fri, Jan 23, 2026 at 06:19:15PM -0800, Mukesh R wrote:
>>>>>>>> On 1/20/26 17:53, Stanislav Kinsburskii wrote:
>>>>>>>>> On Mon, Jan 19, 2026 at 10:42:30PM -0800, Mukesh R wrote:
>>>>>>>>>> From: Mukesh Rathor <mrathor@...ux.microsoft.com>
>>>>>>>>>>
>>>>>>>>>> Upon guest access, in case of missing mmio mapping, the hypervisor
>>>>>>>>>> generates an unmapped gpa intercept. In this path, lookup the PCI
>>>>>>>>>> resource pfn for the guest gpa, and ask the hypervisor to map it
>>>>>>>>>> via hypercall. The PCI resource pfn is maintained by the VFIO driver,
>>>>>>>>>> and obtained via fixup_user_fault call (similar to KVM).
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Mukesh Rathor <mrathor@...ux.microsoft.com>
>>>>>>>>>> ---
>>>>>>>>>>       drivers/hv/mshv_root_main.c | 115 ++++++++++++++++++++++++++++++++++++
>>>>>>>>>>       1 file changed, 115 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
>>>>>>>>>> index 03f3aa9f5541..4c8bc7cd0888 100644
>>>>>>>>>> --- a/drivers/hv/mshv_root_main.c
>>>>>>>>>> +++ b/drivers/hv/mshv_root_main.c
>>>>>>>>>> @@ -56,6 +56,14 @@ struct hv_stats_page {
>>>>>>>>>>       	};
>>>>>>>>>>       } __packed;
>>>>>>>>>> +bool hv_nofull_mmio;   /* don't map entire mmio region upon fault */
>>>>>>>>>> +static int __init setup_hv_full_mmio(char *str)
>>>>>>>>>> +{
>>>>>>>>>> +	hv_nofull_mmio = true;
>>>>>>>>>> +	return 0;
>>>>>>>>>> +}
>>>>>>>>>> +__setup("hv_nofull_mmio", setup_hv_full_mmio);
>>>>>>>>>> +
>>>>>>>>>>       struct mshv_root mshv_root;
>>>>>>>>>>       enum hv_scheduler_type hv_scheduler_type;
>>>>>>>>>> @@ -612,6 +620,109 @@ mshv_partition_region_by_gfn(struct mshv_partition *partition, u64 gfn)
>>>>>>>>>>       }
>>>>>>>>>>       #ifdef CONFIG_X86_64
>>>>>>>>>> +
>>>>>>>>>> +/*
>>>>>>>>>> + * Check if uaddr is for mmio range. If yes, return 0 with mmio_pfn filled in
>>>>>>>>>> + * else just return -errno.
>>>>>>>>>> + */
>>>>>>>>>> +static int mshv_chk_get_mmio_start_pfn(struct mshv_partition *pt, u64 gfn,
>>>>>>>>>> +				       u64 *mmio_pfnp)
>>>>>>>>>> +{
>>>>>>>>>> +	struct vm_area_struct *vma;
>>>>>>>>>> +	bool is_mmio;
>>>>>>>>>> +	u64 uaddr;
>>>>>>>>>> +	struct mshv_mem_region *mreg;
>>>>>>>>>> +	struct follow_pfnmap_args pfnmap_args;
>>>>>>>>>> +	int rc = -EINVAL;
>>>>>>>>>> +
>>>>>>>>>> +	/*
>>>>>>>>>> +	 * Do not allow mem region to be deleted beneath us. VFIO uses
>>>>>>>>>> +	 * useraddr vma to lookup pci bar pfn.
>>>>>>>>>> +	 */
>>>>>>>>>> +	spin_lock(&pt->pt_mem_regions_lock);
>>>>>>>>>> +
>>>>>>>>>> +	/* Get the region again under the lock */
>>>>>>>>>> +	mreg = mshv_partition_region_by_gfn(pt, gfn);
>>>>>>>>>> +	if (mreg == NULL || mreg->type != MSHV_REGION_TYPE_MMIO)
>>>>>>>>>> +		goto unlock_pt_out;
>>>>>>>>>> +
>>>>>>>>>> +	uaddr = mreg->start_uaddr +
>>>>>>>>>> +		((gfn - mreg->start_gfn) << HV_HYP_PAGE_SHIFT);
>>>>>>>>>> +
>>>>>>>>>> +	mmap_read_lock(current->mm);
>>>>>>>>>
>>>>>>>>> Semaphore can't be taken under spinlock.
>>>>>>>
>>>>>>>>
>>>>>>>> Yeah, something didn't feel right here and I meant to recheck, now regret
>>>>>>>> rushing to submit the patch.
>>>>>>>>
>>>>>>>> Rethinking, I think the pt_mem_regions_lock is not needed to protect
>>>>>>>> the uaddr because unmap will properly serialize via the mm lock.
>>>>>>>>
>>>>>>>>
>>>>>>>>>> +	vma = vma_lookup(current->mm, uaddr);
>>>>>>>>>> +	is_mmio = vma ? !!(vma->vm_flags & (VM_IO | VM_PFNMAP)) : 0;
>>>>>>>>>
>>>>>>>>> Why this check is needed again?
>>>>>>>>
>>>>>>>> To make sure region did not change. This check is under lock.
>>>>>>>>
>>>>>>>
>>>>>>> How can this happen? One can't change VMA type without unmapping it
>>>>>>> first. And unmapping it leads to a kernel MMIO region state dangling
>>>>>>> around without corresponding user space mapping.
>>>>>>
>>>>>> Right, and vm_flags would not be mmio expected then.
>>>>>>
>>>>>>> This is similar to dangling pinned regions and should likely be
>>>>>>> addressed the same way by utilizing MMU notifiers to destpoy memoty
>>>>>>> regions is VMA is detached.
>>>>>>
>>>>>> I don't think we need that. Either it succeeds if the region did not
>>>>>> change at all, or just fails.
>>>>>>
>>>>>
>>>>> I'm afraid we do, as if the driver mapped a page with the previous
>>>>> memory region, and then the region is unmapped, the page will stay
>>>>> mapped in the hypervisor, but will be considered free by kernel, which
>>>>> in turn will lead to GPF upn next allocation.
>>>>
>>>> There are no ram pages for mmio regions. Also, we don't do much with
>>>> mmio regions other than tell the hyp about it.
>>>>
>>>
>>> So, are you saying that the hypervisor does not use these pages and only
>>> tracks them? That would make things easier.
>>> However, if we later try to map a GPA that is already mapped, will the
>>> hypervisor return an error?
>>
>> Hypervisor does not return an error.
>>
> 
> So, what happenes if we map a GPA that is already mapped? Does it just
> remap it to the new PFN?

yes, otherwise it would return error, right?

> Thanks,
> Stanislav
> 
>>
>>
>>> Thanks,
>>> Stanislav
>>>
>>>> Thanks,
>>>> -Mukesh
>>>>
>>>>
>>>>> With pinned regions we issue is similar but less impacting: pages can't
>>>>> be released by user space unmapping and thus will be simply leaked, but
>>>>> the system stays intact.
>>>>>
>>>>> MMIO regions are simila to movable region in this regard: they don't
>>>>> reference the user pages, and thus this guest region replaement is a
>>>>> stright wat to kernel panic.
>>>>>
>>>>>>
>>>>>>>>> The region type is stored on the region itself.
>>>>>>>>> And the type is checked on the caller side.
>>>>>>>>>
>>>>>>>>>> +	if (!is_mmio)
>>>>>>>>>> +		goto unlock_mmap_out;
>>>>>>>>>> +
>>>>>>>>>> +	pfnmap_args.vma = vma;
>>>>>>>>>> +	pfnmap_args.address = uaddr;
>>>>>>>>>> +
>>>>>>>>>> +	rc = follow_pfnmap_start(&pfnmap_args);
>>>>>>>>>> +	if (rc) {
>>>>>>>>>> +		rc = fixup_user_fault(current->mm, uaddr, FAULT_FLAG_WRITE,
>>>>>>>>>> +				      NULL);
>>>>>>>>>> +		if (rc)
>>>>>>>>>> +			goto unlock_mmap_out;
>>>>>>>>>> +
>>>>>>>>>> +		rc = follow_pfnmap_start(&pfnmap_args);
>>>>>>>>>> +		if (rc)
>>>>>>>>>> +			goto unlock_mmap_out;
>>>>>>>>>> +	}
>>>>>>>>>> +
>>>>>>>>>> +	*mmio_pfnp = pfnmap_args.pfn;
>>>>>>>>>> +	follow_pfnmap_end(&pfnmap_args);
>>>>>>>>>> +d
>>>>>>>>>> +unlock_mmap_out:
>>>>>>>>>> +	mmap_read_unlock(current->mm);
>>>>>>>>>> +unlock_pt_out:
>>>>>>>>>> +	spin_unlock(&pt->pt_mem_regions_lock);
>>>>>>>>>> +	return rc;
>>>>>>>>>> +}
>>>>>>>>>> +
>>>>>>>>>> +/*
>>>>>>>>>> + * At present, the only unmapped gpa is mmio space. Verify if it's mmio
>>>>>>>>>> + * and resolve if possible.
>>>>>>>>>> + * Returns: True if valid mmio intercept and it was handled, else false
>>>>>>>>>> + */
>>>>>>>>>> +static bool mshv_handle_unmapped_gpa(struct mshv_vp *vp)
>>>>>>>>>> +{
>>>>>>>>>> +	struct hv_message *hvmsg = vp->vp_intercept_msg_page;
>>>>>>>>>> +	struct hv_x64_memory_intercept_message *msg;
>>>>>>>>>> +	union hv_x64_memory_access_info accinfo;
>>>>>>>>>> +	u64 gfn, mmio_spa, numpgs;
>>>>>>>>>> +	struct mshv_mem_region *mreg;
>>>>>>>>>> +	int rc;
>>>>>>>>>> +	struct mshv_partition *pt = vp->vp_partition;
>>>>>>>>>> +
>>>>>>>>>> +	msg = (struct hv_x64_memory_intercept_message *)hvmsg->u.payload;
>>>>>>>>>> +	accinfo = msg->memory_access_info;
>>>>>>>>>> +
>>>>>>>>>> +	if (!accinfo.gva_gpa_valid)
>>>>>>>>>> +		return false;
>>>>>>>>>> +
>>>>>>>>>> +	/* Do a fast check and bail if non mmio intercept */
>>>>>>>>>> +	gfn = msg->guest_physical_address >> HV_HYP_PAGE_SHIFT;
>>>>>>>>>> +	mreg = mshv_partition_region_by_gfn(pt, gfn);
>>>>>>>>>
>>>>>>>>> This call needs to be protected by the spinlock.
>>>>>>>>
>>>>>>>> This is sorta fast path to bail. We recheck under partition lock above.
>>>>>>>>
>>>>>>>
>>>>>>> Accessing the list of regions without lock is unsafe.
>>>>>>
>>>>>> I am not sure why? This check is done by a vcpu thread, so regions
>>>>>> will not have just gone away.
>>>>>>
>>>>>
>>>>> This is shared resources. Multiple VP thread get into this function
>>>>> simultaneously, so there is a race already. But this one we can live
>>>>> with without locking as they don't mutate the list of the regions.
>>>>>
>>>>> The issue happens when VMM adds or removed another region as it mutates
>>>>> the list and races with VP threads doing this lookup.
>>>>>
>>>>> Thanks,
>>>>> Stanislav
>>>>>
>>>>>
>>>>>> Thanks,
>>>>>> -Mukesh
>>>>>>
>>>>>>
>>>>>>> Thanks,
>>>>>>> Stanislav
>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> -Mukesh
>>>>>>>>
>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Stanislav
>>>>>>>>>
>>>>>>>>>> +	if (mreg == NULL || mreg->type != MSHV_REGION_TYPE_MMIO)
>>>>>>>>>> +		return false;
>>>>>>>>>> +
>>>>>>>>>> +	rc = mshv_chk_get_mmio_start_pfn(pt, gfn, &mmio_spa);
>>>>>>>>>> +	if (rc)
>>>>>>>>>> +		return false;
>>>>>>>>>> +
>>>>>>>>>> +	if (!hv_nofull_mmio) {		/* default case */
>>>>>>>>>> +		gfn = mreg->start_gfn;
>>>>>>>>>> +		mmio_spa = mmio_spa - (gfn - mreg->start_gfn);
>>>>>>>>>> +		numpgs = mreg->nr_pages;
>>>>>>>>>> +	} else
>>>>>>>>>> +		numpgs = 1;
>>>>>>>>>> +
>>>>>>>>>> +	rc = hv_call_map_mmio_pages(pt->pt_id, gfn, mmio_spa, numpgs);
>>>>>>>>>> +
>>>>>>>>>> +	return rc == 0;
>>>>>>>>>> +}
>>>>>>>>>> +
>>>>>>>>>>       static struct mshv_mem_region *
>>>>>>>>>>       mshv_partition_region_by_gfn_get(struct mshv_partition *p, u64 gfn)
>>>>>>>>>>       {
>>>>>>>>>> @@ -666,13 +777,17 @@ static bool mshv_handle_gpa_intercept(struct mshv_vp *vp)
>>>>>>>>>>       	return ret;
>>>>>>>>>>       }
>>>>>>>>>> +
>>>>>>>>>>       #else  /* CONFIG_X86_64 */
>>>>>>>>>> +static bool mshv_handle_unmapped_gpa(struct mshv_vp *vp) { return false; }
>>>>>>>>>>       static bool mshv_handle_gpa_intercept(struct mshv_vp *vp) { return false; }
>>>>>>>>>>       #endif /* CONFIG_X86_64 */
>>>>>>>>>>       static bool mshv_vp_handle_intercept(struct mshv_vp *vp)
>>>>>>>>>>       {
>>>>>>>>>>       	switch (vp->vp_intercept_msg_page->header.message_type) {
>>>>>>>>>> +	case HVMSG_UNMAPPED_GPA:
>>>>>>>>>> +		return mshv_handle_unmapped_gpa(vp);
>>>>>>>>>>       	case HVMSG_GPA_INTERCEPT:
>>>>>>>>>>       		return mshv_handle_gpa_intercept(vp);
>>>>>>>>>>       	}
>>>>>>>>>> -- 
>>>>>>>>>> 2.51.2.vfs.0.1
>>>>>>>>>>

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