> -----Original Message-----
> From: Cédric Le Goater <[email protected]>
> Sent: Thursday, September 17, 2026 7:26 PM
> To: Manish Honap <[email protected]>; [email protected]; Ankit Agrawal
> <[email protected]>; [email protected]; [email protected];
> [email protected]; Srirangan Madhavan
> <[email protected]>; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]
> Cc: Krishnakant Jaju <[email protected]>; Vikram Sethi <[email protected]>;
> Zhi Wang <[email protected]>; [email protected]; qemu-
> [email protected]; [email protected]
> Subject: Re: [PATCH v2 05/10] hw/vfio/pci: Back the CXL memory with a
> RAM-device region
>
> External email: Use caution opening links or attachments
>
>
> On 9/16/26 20:44, [email protected] wrote:
> > From: Manish Honap <[email protected]>
> >
> > mmap the kernel's HDM memory region so its host physical pages back a
> > RAM-device MemoryRegion. It stays out of the guest address space until
> > the guest commits its decoder and QEMU learns the GPA to place it at.
> >
> > AI-used-for: code (prototype)
> > Signed-off-by: Manish Honap <[email protected]>
> > ---
> > hw/vfio/pci.c | 34 ++++++++++++++++++++++++++++++++++
> > hw/vfio/pci.h | 1 +
> > 2 files changed, 35 insertions(+)
> >
> > diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c index
> > 10b13f200d..4716266595 100644
> > --- a/hw/vfio/pci.c
> > +++ b/hw/vfio/pci.c
> > @@ -3221,8 +3221,11 @@ bool vfio_pci_populate_device(VFIOPCIDevice
> *vdev, Error **errp)
> > return true;
> > }
> >
> > +static void vfio_cxl_teardown(VFIOPCIDevice *vdev);
> > +
> > void vfio_pci_put_device(VFIOPCIDevice *vdev)
> > {
> > + vfio_cxl_teardown(vdev);
> > vfio_display_finalize(vdev);
> > vfio_bars_finalize(vdev);
> > vfio_cpr_pci_unregister_device(vdev);
> > @@ -3696,11 +3699,42 @@ static bool vfio_cxl_setup(VFIOPCIDevice
> *vdev, Error **errp)
> > return false;
> > }
> >
> > + /*
> > + * The HDM memory is host physical. Set up the region, which installs
> > the
> > + * fd read/write path, and mmap it for direct guest access; it is
> > added to
> > + * the guest address space only once the guest commits its endpoint
> decoder.
> > + */
> > + if (vfio_region_setup(OBJECT(vdev), vbasedev, &cxl->mem_region,
> > + cxl->mem_region_index, "cxl-mem", errp)) {
> > + return false;
> > + }
> > + if (vfio_region_mmap(&cxl->mem_region)) {
> > + /*
> > + * Without mmap the region falls back to the kernel's fd read/write
> > + * path, which works but traps every access. Warn rather than fail.
> > + */
> > + warn_report("vfio-cxl: %s: failed to mmap the HDM memory region; "
> > + "performance may be slow", vbasedev->name);
> > + }
>
> vfio_cxl_setup calls vfio_region_setup() then vfio_region_mmap() back-to-
> back when other 'normal' BARs split these calls across
> vfio_populate_device() and vfio_bar_register(). I guess it is fine for CXL
> since it
> is not a PCI BAR.
>
Yes, I also had the same reasoning for this part.
> > cxl->enabled = true;
> >
> > return true;
> > }
> >
> > +static void vfio_cxl_teardown(VFIOPCIDevice *vdev) {
> > + VFIOCXL *cxl = &vdev->cxl;
> > +
> > + if (!cxl->enabled) {
> > + return;
> > + }
> > + if (cxl->mem_region.mem) {
> > + vfio_region_exit(&cxl->mem_region);
> > + vfio_region_finalize(&cxl->mem_region);
>
> However, the tear down should be split between :
>
> 1. vfio_exitfn (unrealize)
> drops mmaps, removes subregions and removes references so the
> MR refcount can reach zero.
> 2. vfio_pci_finalize (instance_finalize)
> frees the MemoryRegion.
>
okay, I will move this as suggested.
> Thanks,
>
> C.
>
> > + }
> > +}
> > +
> > static void vfio_pci_realize(PCIDevice *pdev, Error **errp)
> > {
> > ERRP_GUARD();
> > diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h index
> > 7fdd695704..06d15e807d 100644
> > --- a/hw/vfio/pci.h
> > +++ b/hw/vfio/pci.h
> > @@ -134,6 +134,7 @@ typedef struct VFIOCXL {
> > uint32_t comp_bar; /* component BAR carrying that block
> > */
> > uint64_t hdm_offset; /* block offset within the component
> > BAR */
> > uint64_t dpa_size; /* size of the HDM memory region */
> > + VFIORegion mem_region; /* HDM memory, mapped at committed
> GPA */
> > } VFIOCXL;
> >
> > struct VFIOPCIDevice {
