Re: [PATCH v8 1/1] vfio/nvgrace-gpu: Add Blackwell-Next GPU readiness check via CXL DVSEC

[Cédric Le Goater]

Hello Ankit,

On 6/2/26 08:30, Ankit Agrawal wrote:
> Add a CXL DVSEC-based readiness check for Blackwell-Next GPUs alongside
> the existing legacy BAR0 polling path. The CXL Device DVSEC offset is
> discovered at probe time. Probe, fault and read/write paths then branch
> on that to use either the legacy BAR0 polling or the CXL DVSEC polling.
>
> The CXL path polls Memory_Active, requiring MEM_INFO_VALID within 1s and
> MEM_ACTIVE within Memory_Active_Timeout (up to 256s) as per CXL spec r4.0
> sec 8.1.3.8.2. Given the long worst-case wait, the CXL poll runs outside
> memory_lock with only a quick readiness check is done under the lock.
>
> The poll loops sleep with schedule_timeout_killable() and return -EINTR
> on a fatal signal. This avoids hung-task panics during the long
> uninterruptible wait. Extend this to the legacy based wait as well for
> improvement.
>
> In the fault handler the wait runs locklessly before memory_lock. If a
> reset races in, the in-lock recheck returns -EAGAIN and the wait is
> retried rather than returning a spurious VM_FAULT_SIGBUS.
>
> Add PCI_DVSEC_CXL_MEM_ACTIVE_TIMEOUT to pci_regs.h for the timeout field.
>
> Cc: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> Suggested-by: Alex Williamson <alex@xxxxxxxxxxx>
> Signed-off-by: Ankit Agrawal <ankita@xxxxxxxxxx>
> ---
>   drivers/vfio/pci/nvgrace-gpu/main.c | 162 +++++++++++++++++++++++++---
>   include/uapi/linux/pci_regs.h       |   1 +
>   2 files changed, 151 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c
> index fa056b69f899..4829ae29cff0 100644
> --- a/drivers/vfio/pci/nvgrace-gpu/main.c
> +++ b/drivers/vfio/pci/nvgrace-gpu/main.c
> @@ -3,10 +3,13 @@
>    * Copyright (c) 2024, NVIDIA CORPORATION & AFFILIATES. All rights reserved
>    */
>   
> +#include <linux/bitfield.h>
>   #include <linux/sizes.h>
> +#include <linux/time64.h>
>   #include <linux/vfio_pci_core.h>
>   #include <linux/delay.h>
>   #include <linux/jiffies.h>
> +#include <linux/sched.h>
>   #include <linux/pci-p2pdma.h>
>   #include <linux/pm_runtime.h>
>   #include <linux/memory-failure.h>
> @@ -64,6 +67,8 @@ struct nvgrace_gpu_pci_core_device {
>   	bool has_mig_hw_bug;
>   	/* GPU has just been reset */
>   	bool reset_done;
> +	/* CXL Device DVSEC offset; 0 if not present (legacy GB path) */
> +	int cxl_dvsec;
>   };
>   
>   static void nvgrace_gpu_init_fake_bar_emu_regs(struct vfio_device *core_vdev)
> @@ -242,7 +247,7 @@ static void nvgrace_gpu_close_device(struct vfio_device *core_vdev)
>   	vfio_pci_core_close_device(core_vdev);
>   }
>   
> -static int nvgrace_gpu_wait_device_ready(void __iomem *io)
> +static int nvgrace_gpu_wait_device_ready_legacy(void __iomem *io)
>   {
>   	unsigned long timeout = jiffies + msecs_to_jiffies(POLL_TIMEOUT_MS);
>   
> @@ -250,16 +255,97 @@ static int nvgrace_gpu_wait_device_ready(void __iomem *io)
>   		if ((ioread32(io + C2C_LINK_BAR0_OFFSET) == STATUS_READY) &&
>   		    (ioread32(io + HBM_TRAINING_BAR0_OFFSET) == STATUS_READY))
>   			return 0;
> -		msleep(POLL_QUANTUM_MS);
> +		if (schedule_timeout_killable(msecs_to_jiffies(POLL_QUANTUM_MS)))
> +			return -EINTR;
>   	} while (!time_after(jiffies, timeout));
>   
>   	return -ETIME;
>   }
>   
> +/*
> + * Decode the 3-bit Memory_Active_Timeout field from CXL DVSEC Range 1 Low
> + * (bits 15:13) into milliseconds. Encoding per CXL spec r4.0 sec 8.1.3.8.2:
> + * 000b = 1s, 001b = 4s, 010b = 16s, 011b = 64s, 100b = 256s,
> + * 101b-111b = reserved (clamped to 256s).
> + */
> +static inline unsigned long cxl_mem_active_timeout_ms(u8 timeout)
> +{
> +	return MSEC_PER_SEC << (2 * min_t(u8, timeout, 4));
> +}
> +
> +/*
> + * Check if CXL DVSEC reports memory as valid and active.
> + */
> +static inline bool cxl_dvsec_mem_is_active(u32 status)
> +{
> +	return (status & PCI_DVSEC_CXL_MEM_INFO_VALID) &&
> +	       (status & PCI_DVSEC_CXL_MEM_ACTIVE);
> +}
> +
> +static int nvgrace_gpu_test_device_ready_cxl(struct nvgrace_gpu_pci_core_device *nvdev,
> +					     u32 *status)
> +{
> +	struct pci_dev *pdev = nvdev->core_device.pdev;
> +	int cxl_dvsec = nvdev->cxl_dvsec;
> +	u32 val;
> +
> +	pci_read_config_dword(pdev,
> +			      cxl_dvsec + PCI_DVSEC_CXL_RANGE_SIZE_LOW(0),
> +			      &val);
> +
> +	if (val == ~0U)
> +		return -ENODEV;
> +
> +	if (status)
> +		*status = val;
> +
> +	if (cxl_dvsec_mem_is_active(val))
> +		return 0;
> +
> +	return -EAGAIN;
> +}
> +
> +/*
> + * As per CXL spec r4.0 sec 8.1.3.8.2, MEM_INFO_VALID needs to be set
> + * within 1s and MEM_ACTIVE within Memory_Active_Timeout (up to ~256s)
> + * after reset and bootup.
> + */
> +static int nvgrace_gpu_wait_device_ready_cxl(struct nvgrace_gpu_pci_core_device *nvdev)
> +{
> +	unsigned long deadline = jiffies + msecs_to_jiffies(POLL_QUANTUM_MS);
> +	bool active_phase = false;
> +	u32 status;
> +	int ret;
> +
> +	for (;;) {
> +		ret = nvgrace_gpu_test_device_ready_cxl(nvdev, &status);
> +		if (ret != -EAGAIN)
> +			return ret;
> +
> +		if (!active_phase && (status & PCI_DVSEC_CXL_MEM_INFO_VALID)) {
> +			u8 t = FIELD_GET(PCI_DVSEC_CXL_MEM_ACTIVE_TIMEOUT, status);
> +
> +			deadline = jiffies +
> +				   msecs_to_jiffies(cxl_mem_active_timeout_ms(t));
> +			active_phase = true;
> +		}
> +
> +		if (time_after(jiffies, deadline))
> +			return -ETIME;
> +
> +		if (schedule_timeout_killable(msecs_to_jiffies(POLL_QUANTUM_MS)))
> +			return -EINTR;
> +	}
> +}
> +
>   /*
>    * If the GPU memory is accessed by the CPU while the GPU is not ready
>    * after reset, it can cause harmless corrected RAS events to be logged.
>    * Make sure the GPU is ready before establishing the mappings.
> + *
> + * Since the CXL polling wait could take 256s, it happens outside
> + * memory_lock. Only do quick readiness check under the lock. Legacy
> + * keeps the in-lock poll.
>    */
>   static int
>   nvgrace_gpu_check_device_ready(struct nvgrace_gpu_pci_core_device *nvdev)
> @@ -275,7 +361,10 @@ nvgrace_gpu_check_device_ready(struct nvgrace_gpu_pci_core_device *nvdev)
>   	if (!__vfio_pci_memory_enabled(vdev))
>   		return -EIO;
>   
> -	ret = nvgrace_gpu_wait_device_ready(vdev->barmap[0]);
> +	if (nvdev->cxl_dvsec)
> +		ret = nvgrace_gpu_test_device_ready_cxl(nvdev, NULL);
> +	else
> +		ret = nvgrace_gpu_wait_device_ready_legacy(vdev->barmap[0]);
>   	if (ret)
>   		return ret;
>   
> @@ -313,9 +402,33 @@ static vm_fault_t nvgrace_gpu_vfio_pci_huge_fault(struct vm_fault *vmf,
>   	pfn = PHYS_PFN(memregion->memphys) + addr_to_pgoff(vma, addr);
>   
>   	if (is_aligned_for_order(vma, addr, pfn, order)) {
> +		/*
> +		 * Exit early under memory_lock to avoid a potentially lengthy
> +		 * device readiness wait on a runtime-suspended device. Any
> +		 * race after the lock is dropped is benign as the re-check
> +		 * inside the scoped guard below catches it.
> +		 */
> +		scoped_guard(rwsem_read, &vdev->memory_lock) {
> +			if (vdev->pm_runtime_engaged)
> +				return VM_FAULT_SIGBUS;
> +		}
> +
> +retry:
> +		if (nvdev->cxl_dvsec && READ_ONCE(nvdev->reset_done) &&
> +		    nvgrace_gpu_wait_device_ready_cxl(nvdev))
> +			return VM_FAULT_SIGBUS;
> +
>   		scoped_guard(rwsem_read, &vdev->memory_lock) {
> -			if (vdev->pm_runtime_engaged ||
> -			    nvgrace_gpu_check_device_ready(nvdev))
> +			int rc;
> +
> +			if (vdev->pm_runtime_engaged)
> +				return VM_FAULT_SIGBUS;
> +
> +			/* Re-run the wait if a reset raced us, not SIGBUS. */
> +			rc = nvgrace_gpu_check_device_ready(nvdev);
> +			if (rc == -EAGAIN)
> +				goto retry;
> +			if (rc)
>   				return VM_FAULT_SIGBUS;
>   
>   			ret = vfio_pci_vmf_insert_pfn(vdev, vmf, pfn, order);
> @@ -712,6 +825,12 @@ nvgrace_gpu_read_mem(struct nvgrace_gpu_pci_core_device *nvdev,
>   	else
>   		mem_count = min(count, memregion->memlength - (size_t)offset);
>   
> +	if (nvdev->cxl_dvsec && READ_ONCE(nvdev->reset_done)) {
> +		ret = nvgrace_gpu_wait_device_ready_cxl(nvdev);
> +		if (ret)
> +			return ret;
> +	}
> +
>   	scoped_guard(rwsem_read, &vdev->memory_lock) {
>   		ret = nvgrace_gpu_check_device_ready(nvdev);
>   		if (ret)
> @@ -846,6 +965,12 @@ nvgrace_gpu_write_mem(struct nvgrace_gpu_pci_core_device *nvdev,
>   	 */
>   	mem_count = min(count, memregion->memlength - (size_t)offset);
>   
> +	if (nvdev->cxl_dvsec && READ_ONCE(nvdev->reset_done)) {
> +		ret = nvgrace_gpu_wait_device_ready_cxl(nvdev);
> +		if (ret)
> +			return ret;
> +	}
> +
>   	scoped_guard(rwsem_read, &vdev->memory_lock) {
>   		ret = nvgrace_gpu_check_device_ready(nvdev);
>   		if (ret)
> @@ -1143,14 +1268,24 @@ static bool nvgrace_gpu_has_mig_hw_bug(struct pci_dev *pdev)
>    * is beneficial to make the check to ensure the device is in an
>    * expected state.
>    *
> - * Ensure that the BAR0 region is enabled before accessing the
> + * On Blackwell-Next systems, memory readiness is determined via the
> + * CXL Device DVSEC in PCI config space and does not require BAR0.
> + * For the legacy path, ensure BAR0 is enabled before accessing the
>    * registers.
>    */
> -static int nvgrace_gpu_probe_check_device_ready(struct pci_dev *pdev)
> +static int nvgrace_gpu_probe_check_device_ready(struct nvgrace_gpu_pci_core_device *nvdev)
>   {
> +	struct pci_dev *pdev = nvdev->core_device.pdev;
>   	void __iomem *io;
>   	int ret;
>   
> +	/*
> +	 * Note that the worst-case wait here is ~256s (vs ~30s on the
> +	 * legacy path) and may block device unbind/sysfs for the duration.
> +	 */
> +	if (nvdev->cxl_dvsec)
> +		return nvgrace_gpu_wait_device_ready_cxl(nvdev);
> +
>   	ret = pci_enable_device(pdev);
>   	if (ret)
>   		return ret;
> @@ -1165,7 +1300,7 @@ static int nvgrace_gpu_probe_check_device_ready(struct pci_dev *pdev)
>   		goto iomap_exit;
>   	}
>   
> -	ret = nvgrace_gpu_wait_device_ready(io);
> +	ret = nvgrace_gpu_wait_device_ready_legacy(io);
>   
>   	pci_iounmap(pdev, io);
>   iomap_exit:
> @@ -1183,10 +1318,6 @@ static int nvgrace_gpu_probe(struct pci_dev *pdev,
>   	u64 memphys, memlength;
>   	int ret;
>   
> -	ret = nvgrace_gpu_probe_check_device_ready(pdev);
> -	if (ret)
> -		return ret;
> -
>   	ret = nvgrace_gpu_fetch_memory_property(pdev, &memphys, &memlength);
>   	if (!ret)
>   		ops = &nvgrace_gpu_pci_ops;
> @@ -1196,6 +1327,13 @@ static int nvgrace_gpu_probe(struct pci_dev *pdev,
>   	if (IS_ERR(nvdev))
>   		return PTR_ERR(nvdev);
>   
> +	nvdev->cxl_dvsec = pci_find_dvsec_capability(pdev, PCI_VENDOR_ID_CXL,
> +						     PCI_DVSEC_CXL_DEVICE);
> +
> +	ret = nvgrace_gpu_probe_check_device_ready(nvdev);
> +	if (ret)
> +		goto out_put_vdev;
> +
>   	dev_set_drvdata(&pdev->dev, &nvdev->core_device);
>   
>   	if (ops == &nvgrace_gpu_pci_ops) {

Shouldn't an update be made to nvgrace_gpu_vfio_pci_table?

Thanks,

C.

> diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
> index 14f634ab9350..718fb630f5bb 100644
> --- a/include/uapi/linux/pci_regs.h
> +++ b/include/uapi/linux/pci_regs.h
> @@ -1357,6 +1357,7 @@
>   #define  PCI_DVSEC_CXL_RANGE_SIZE_LOW(i)		(0x1C + (i * 0x10))
>   #define   PCI_DVSEC_CXL_MEM_INFO_VALID			_BITUL(0)
>   #define   PCI_DVSEC_CXL_MEM_ACTIVE			_BITUL(1)
> +#define   PCI_DVSEC_CXL_MEM_ACTIVE_TIMEOUT		__GENMASK(15, 13)
>   #define   PCI_DVSEC_CXL_MEM_SIZE_LOW			__GENMASK(31, 28)
>   #define  PCI_DVSEC_CXL_RANGE_BASE_HIGH(i)		(0x20 + (i * 0x10))
>   #define  PCI_DVSEC_CXL_RANGE_BASE_LOW(i)		(0x24 + (i * 0x10))