Re: [PATCH v0 15/15] mshv: Populate mmio mappings for PCI passthru

Stanislav Kinsburskii
2026-02-02T16:31:01Z

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@linux.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@linux.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?

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
> > > > > > >