Message-ID: <596c9549-9edc-91f3-7473-e206ddc68e76@linux.microsoft.com>
Date: Wed, 4 Feb 2026 14:52:54 -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 2/2/26 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.



> 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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