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([2a01:e0a:1292:d530:e939:7a09:c368:e5d5]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6aa1d2727bfsm3365323a12.20.2026.09.17.06.56.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 17 Sep 2026 06:56:15 -0700 (PDT) Message-ID: <0991dccb-a123-471c-8a72-e4194da6c596@redhat.com> Date: Thu, 17 Sep 2026 15:56:13 +0200 Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 05/10] hw/vfio/pci: Back the CXL memory with a RAM-device region To: mhonap@nvidia.com, alex@shazbot.org, ankita@nvidia.com, jic23@kernel.org, dave.jiang@intel.com, alejandro.lucero-palau@amd.com, smadhavan@nvidia.com, pierrick.bouvier@oss.qualcomm.com, mst@redhat.com, imammedo@redhat.com, anisinha@redhat.com, pbonzini@redhat.com, eric.auger@redhat.com, peter.maydell@linaro.org, richard.henderson@linaro.org, cohuck@redhat.com Cc: kjaju@nvidia.com, vsethi@nvidia.com, zhiw@nvidia.com, qemu-devel@nongnu.org, qemu-arm@nongnu.org, linux-cxl@vger.kernel.org References: <20260916184412.3825713-1-mhonap@nvidia.com> <20260916184412.3825713-6-mhonap@nvidia.com> From: =?UTF-8?Q?C=C3=A9dric_Le_Goater?= Autocrypt: addr=clg@redhat.com; keydata= xsFNBFu8o3UBEADP+oJVJaWm5vzZa/iLgpBAuzxSmNYhURZH+guITvSySk30YWfLYGBWQgeo 8NzNXBY3cH7JX3/a0jzmhDc0U61qFxVgrPqs1PQOjp7yRSFuDAnjtRqNvWkvlnRWLFq4+U5t yzYe4SFMjFb6Oc0xkQmaK2flmiJNnnxPttYwKBPd98WfXMmjwAv7QfwW+OL3VlTPADgzkcqj 53bfZ4VblAQrq6Ctbtu7JuUGAxSIL3XqeQlAwwLTfFGrmpY7MroE7n9Rl+hy/kuIrb/TO8n0 ZxYXvvhT7OmRKvbYuc5Jze6o7op/bJHlufY+AquYQ4dPxjPPVUT/DLiUYJ3oVBWFYNbzfOrV RxEwNuRbycttMiZWxgflsQoHF06q/2l4ttS3zsV4TDZudMq0TbCH/uJFPFsbHUN91qwwaN/+ gy1j7o6aWMz+Ib3O9dK2M/j/O/Ube95mdCqN4N/uSnDlca3YDEWrV9jO1mUS/ndOkjxa34ia 70FjwiSQAsyIwqbRO3CGmiOJqDa9qNvd2TJgAaS2WCw/TlBALjVQ7AyoPEoBPj31K74Wc4GS Rm+FSch32ei61yFu6ACdZ12i5Edt+To+hkElzjt6db/UgRUeKfzlMB7PodK7o8NBD8outJGS tsL2GRX24QvvBuusJdMiLGpNz3uqyqwzC5w0Fd34E6G94806fwARAQABzSJDw6lkcmljIExl IEdvYXRlciA8Y2xnQHJlZGhhdC5jb20+wsGRBBMBCAA7FiEEoPZlSPBIlev+awtgUaNDx8/7 7KEFAmTLlVECGwMFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AACgkQUaNDx8/77KG0eg// S0zIzTcxkrwJ/9XgdcvVTnXLVF9V4/tZPfB7sCp8rpDCEseU6O0TkOVFoGWM39sEMiQBSvyY lHrP7p7E/JYQNNLh441MfaX8RJ5Ul3btluLapm8oHp/vbHKV2IhLcpNCfAqaQKdfk8yazYhh EdxTBlzxPcu+78uE5fF4wusmtutK0JG0sAgq0mHFZX7qKG6LIbdLdaQalZ8CCFMKUhLptW71 xe+aNrn7hScBoOj2kTDRgf9CE7svmjGToJzUxgeh9mIkxAxTu7XU+8lmL28j2L5uNuDOq9vl hM30OT+pfHmyPLtLK8+GXfFDxjea5hZLF+2yolE/ATQFt9AmOmXC+YayrcO2ZvdnKExZS1o8 VUKpZgRnkwMUUReaF/mTauRQGLuS4lDcI4DrARPyLGNbvYlpmJWnGRWCDguQ/LBPpbG7djoy k3NlvoeA757c4DgCzggViqLm0Bae320qEc6z9o0X0ePqSU2f7vcuWN49Uhox5kM5L86DzjEQ RHXndoJkeL8LmHx8DM+kx4aZt0zVfCHwmKTkSTQoAQakLpLte7tWXIio9ZKhUGPv/eHxXEoS 0rOOAZ6np1U/xNR82QbF9qr9TrTVI3GtVe7Vxmff+qoSAxJiZQCo5kt0YlWwti2fFI4xvkOi V7lyhOA3+/3oRKpZYQ86Frlo61HU3r6d9wzOwU0EW7yjdQEQALyDNNMw/08/fsyWEWjfqVhW pOOrX2h+z4q0lOHkjxi/FRIRLfXeZjFfNQNLSoL8j1y2rQOs1j1g+NV3K5hrZYYcMs0xhmrZ KXAHjjDx7FW3sG3jcGjFW5Xk4olTrZwFsZVUcP8XZlArLmkAX3UyrrXEWPSBJCXxDIW1hzwp bV/nVbo/K9XBptT/wPd+RPiOTIIRptjypGY+S23HYBDND3mtfTz/uY0Jytaio9GETj+fFis6 TxFjjbZNUxKpwftu/4RimZ7qL+uM1rG1lLWc9SPtFxRQ8uLvLOUFB1AqHixBcx7LIXSKZEFU CSLB2AE4wXQkJbApye48qnZ09zc929df5gU6hjgqV9Gk1rIfHxvTsYltA1jWalySEScmr0iS YBZjw8Nbd7SxeomAxzBv2l1Fk8fPzR7M616dtb3Z3HLjyvwAwxtfGD7VnvINPbzyibbe9c6g LxYCr23c2Ry0UfFXh6UKD83d5ybqnXrEJ5n/t1+TLGCYGzF2erVYGkQrReJe8Mld3iGVldB7 JhuAU1+d88NS3aBpNF6TbGXqlXGF6Yua6n1cOY2Yb4lO/mDKgjXd3aviqlwVlodC8AwI0Sdu jWryzL5/AGEU2sIDQCHuv1QgzmKwhE58d475KdVX/3Vt5I9kTXpvEpfW18TjlFkdHGESM/Jx IqVsqvhAJkalABEBAAHCwV8EGAECAAkFAlu8o3UCGwwACgkQUaNDx8/77KEhwg//WqVopd5k 8hQb9VVdk6RQOCTfo6wHhEqgjbXQGlaxKHoXywEQBi8eULbeMQf5l4+tHJWBxswQ93IHBQjK yKyNr4FXseUI5O20XVNYDJZUrhA4yn0e/Af0IX25d94HXQ5sMTWr1qlSK6Zu79lbH3R57w9j hQm9emQEp785ui3A5U2Lqp6nWYWXz0eUZ0Tad2zC71Gg9VazU9MXyWn749s0nXbVLcLS0yop s302Gf3ZmtgfXTX/W+M25hiVRRKCH88yr6it+OMJBUndQVAA/fE9hYom6t/zqA248j0QAV/p LHH3hSirE1mv+7jpQnhMvatrwUpeXrOiEw1nHzWCqOJUZ4SY+HmGFW0YirWV2mYKoaGO2YBU wYF7O9TI3GEEgRMBIRT98fHa0NPwtlTktVISl73LpgVscdW8yg9Gc82oe8FzU1uHjU8b10lU XOMHpqDDEV9//r4ZhkKZ9C4O+YZcTFu+mvAY3GlqivBNkmYsHYSlFsbxc37E1HpTEaSWsGfA HQoPn9qrDJgsgcbBVc1gkUT6hnxShKPp4PlsZVMNjvPAnr5TEBgHkk54HQRhhwcYv1T2QumQ izDiU6iOrUzBThaMhZO3i927SG2DwWDVzZltKrCMD1aMPvb3NU8FOYRhNmIFR3fcalYr+9gD uVKe8BVz4atMOoktmt0GWTOC8P4= In-Reply-To: <20260916184412.3825713-6-mhonap@nvidia.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 7ULNNPv9eI5BqB6L09vh2W_F5tk8VdEG4nOBCQm8L18_1789653376 X-Mimecast-Originator: redhat.com Content-Language: en-US, fr Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/16/26 20:44, mhonap@nvidia.com wrote: > From: Manish Honap > > 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 > --- > 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. > 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. 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 {