Re: [PATCH 11/14] vfio: add vfio_pci_config_space_read/write()

[Tomita Moeko <tomitamoeko@gmail.com>]

On 4/9/25 21:48, John Levon wrote:
> Add these helpers that access config space and return an -errno style
> return.
> 
> Signed-off-by: John Levon <john.levon@nutanix.com>
> ---
>  hw/vfio/pci.c | 134 ++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 91 insertions(+), 43 deletions(-)
> 
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index ddeee33aa9..c3842d2f8d 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -964,6 +964,28 @@ static void vfio_pci_load_rom(VFIOPCIDevice *vdev)
>      }
>  }
>  
> +/* "Raw" read of underlying config space. */
> +static int vfio_pci_config_space_read(VFIOPCIDevice *vdev, off_t offset,
> +                                      uint32_t size, void *data)

Returning ssize_t here might be better here to avoid casting issues,
though we would never read/write something exceeds INT32_MAX.

Thanks,
Moeko

> +{
> +    ssize_t ret;
> +
> +    ret = pread(vdev->vbasedev.fd, data, size, vdev->config_offset + offset);
> +
> +    return ret < 0 ? -errno : (int)ret;
> +}
> +
> +/* "Raw" write of underlying config space. */
> +static int vfio_pci_config_space_write(VFIOPCIDevice *vdev, off_t offset,
> +                                       uint32_t size, void *data)
> +{
> +    ssize_t ret;
> +
> +    ret = pwrite(vdev->vbasedev.fd, data, size, vdev->config_offset + offset);
> +
> +    return ret < 0 ? -errno : (int)ret;
> +}
> +
>  static uint64_t vfio_rom_read(void *opaque, hwaddr addr, unsigned size)
>  {
>      VFIOPCIDevice *vdev = opaque;
> @@ -1016,10 +1038,9 @@ static const MemoryRegionOps vfio_rom_ops = {
>  
>  static void vfio_pci_size_rom(VFIOPCIDevice *vdev)
>  {
> +    VFIODevice *vbasedev = &vdev->vbasedev;
>      uint32_t orig, size = cpu_to_le32((uint32_t)PCI_ROM_ADDRESS_MASK);
> -    off_t offset = vdev->config_offset + PCI_ROM_ADDRESS;
>      char *name;
> -    int fd = vdev->vbasedev.fd;
>  
>      if (vdev->pdev.romfile || !vdev->pdev.rom_bar) {
>          /* Since pci handles romfile, just print a message and return */
> @@ -1036,11 +1057,12 @@ static void vfio_pci_size_rom(VFIOPCIDevice *vdev)
>       * Use the same size ROM BAR as the physical device.  The contents
>       * will get filled in later when the guest tries to read it.
>       */
> -    if (pread(fd, &orig, 4, offset) != 4 ||
> -        pwrite(fd, &size, 4, offset) != 4 ||
> -        pread(fd, &size, 4, offset) != 4 ||
> -        pwrite(fd, &orig, 4, offset) != 4) {
> -        error_report("%s(%s) failed: %m", __func__, vdev->vbasedev.name);
> +    if (vfio_pci_config_space_read(vdev, PCI_ROM_ADDRESS, 4, &orig) != 4 ||
> +        vfio_pci_config_space_write(vdev, PCI_ROM_ADDRESS, 4, &size) != 4 ||
> +        vfio_pci_config_space_read(vdev, PCI_ROM_ADDRESS, 4, &size) != 4 ||
> +        vfio_pci_config_space_write(vdev, PCI_ROM_ADDRESS, 4, &orig) != 4) {
> +
> +        error_report("%s(%s) ROM access failed", __func__, vbasedev->name);
>          return;
>      }
>  
> @@ -1220,6 +1242,7 @@ static void vfio_sub_page_bar_update_mapping(PCIDevice *pdev, int bar)
>  uint32_t vfio_pci_read_config(PCIDevice *pdev, uint32_t addr, int len)
>  {
>      VFIOPCIDevice *vdev = VFIO_PCI_BASE(pdev);
> +    VFIODevice *vbasedev = &vdev->vbasedev;
>      uint32_t emu_bits = 0, emu_val = 0, phys_val = 0, val;
>  
>      memcpy(&emu_bits, vdev->emulated_config_bits + addr, len);
> @@ -1232,12 +1255,13 @@ uint32_t vfio_pci_read_config(PCIDevice *pdev, uint32_t addr, int len)
>      if (~emu_bits & (0xffffffffU >> (32 - len * 8))) {
>          ssize_t ret;
>  
> -        ret = pread(vdev->vbasedev.fd, &phys_val, len,
> -                    vdev->config_offset + addr);
> +        ret = vfio_pci_config_space_read(vdev, addr, len, &phys_val);
>          if (ret != len) {
> -            error_report("%s(%s, 0x%x, 0x%x) failed: %m",
> -                         __func__, vdev->vbasedev.name, addr, len);
> -            return -errno;
> +            const char *err = ret < 0 ? strerror(-ret) : "short read";
> +
> +            error_report("%s(%s, 0x%x, 0x%x) failed: %s",
> +                         __func__, vbasedev->name, addr, len, err);
> +            return -1;
>          }
>          phys_val = le32_to_cpu(phys_val);
>      }
> @@ -1253,15 +1277,19 @@ void vfio_pci_write_config(PCIDevice *pdev,
>                             uint32_t addr, uint32_t val, int len)
>  {
>      VFIOPCIDevice *vdev = VFIO_PCI_BASE(pdev);
> +    VFIODevice *vbasedev = &vdev->vbasedev;
>      uint32_t val_le = cpu_to_le32(val);
> +    int ret;
>  
>      trace_vfio_pci_write_config(vdev->vbasedev.name, addr, val, len);
>  
>      /* Write everything to VFIO, let it filter out what we can't write */
> -    if (pwrite(vdev->vbasedev.fd, &val_le, len, vdev->config_offset + addr)
> -                != len) {
> -        error_report("%s(%s, 0x%x, 0x%x, 0x%x) failed: %m",
> -                     __func__, vdev->vbasedev.name, addr, val, len);
> +    ret = vfio_pci_config_space_write(vdev, addr, len, &val_le);
> +    if (ret != len) {
> +        const char *err = ret < 0 ? strerror(-ret) : "short write";
> +
> +        error_report("%s(%s, 0x%x, 0x%x, 0x%x) failed: %s",
> +                     __func__, vbasedev->name, addr, val, len, err);
>      }
>  
>      /* MSI/MSI-X Enabling/Disabling */
> @@ -1349,9 +1377,12 @@ static bool vfio_msi_setup(VFIOPCIDevice *vdev, int pos, Error **errp)
>      int ret, entries;
>      Error *err = NULL;
>  
> -    if (pread(vdev->vbasedev.fd, &ctrl, sizeof(ctrl),
> -              vdev->config_offset + pos + PCI_CAP_FLAGS) != sizeof(ctrl)) {
> -        error_setg_errno(errp, errno, "failed reading MSI PCI_CAP_FLAGS");
> +    ret = vfio_pci_config_space_read(vdev, pos + PCI_CAP_FLAGS,
> +                                     sizeof(ctrl), &ctrl);
> +    if (ret != sizeof(ctrl)) {
> +        const char *errmsg = ret < 0 ? strerror(-ret) : "short read";
> +
> +        error_setg(errp, "failed reading MSI PCI_CAP_FLAGS: %s", errmsg);
>          return false;
>      }
>      ctrl = le16_to_cpu(ctrl);
> @@ -1558,30 +1589,39 @@ static bool vfio_msix_early_setup(VFIOPCIDevice *vdev, Error **errp)
>      uint8_t pos;
>      uint16_t ctrl;
>      uint32_t table, pba;
> -    int ret, fd = vdev->vbasedev.fd;
>      struct vfio_irq_info irq_info;
>      VFIOMSIXInfo *msix;
> +    int ret;
>  
>      pos = pci_find_capability(&vdev->pdev, PCI_CAP_ID_MSIX);
>      if (!pos) {
>          return true;
>      }
>  
> -    if (pread(fd, &ctrl, sizeof(ctrl),
> -              vdev->config_offset + pos + PCI_MSIX_FLAGS) != sizeof(ctrl)) {
> -        error_setg_errno(errp, errno, "failed to read PCI MSIX FLAGS");
> +    ret = vfio_pci_config_space_read(vdev, pos + PCI_MSIX_FLAGS,
> +                                     sizeof(ctrl), &ctrl);
> +    if (ret != sizeof(ctrl)) {
> +        const char *err = ret < 0 ? strerror(-ret) : "short read";
> +
> +        error_setg(errp, "failed to read PCI MSIX FLAGS: %s", err);
>          return false;
>      }
>  
> -    if (pread(fd, &table, sizeof(table),
> -              vdev->config_offset + pos + PCI_MSIX_TABLE) != sizeof(table)) {
> -        error_setg_errno(errp, errno, "failed to read PCI MSIX TABLE");
> +    ret = vfio_pci_config_space_read(vdev, pos + PCI_MSIX_TABLE,
> +                                     sizeof(table), &table);
> +    if (ret != sizeof(table)) {
> +        const char *err = ret < 0 ? strerror(-ret) : "short read";
> +
> +        error_setg(errp, "failed to read PCI MSIX TABLE: %s", err);
>          return false;
>      }
>  
> -    if (pread(fd, &pba, sizeof(pba),
> -              vdev->config_offset + pos + PCI_MSIX_PBA) != sizeof(pba)) {
> -        error_setg_errno(errp, errno, "failed to read PCI MSIX PBA");
> +    ret = vfio_pci_config_space_read(vdev, pos + PCI_MSIX_PBA,
> +                                     sizeof(pba), &pba);
> +    if (ret != sizeof(pba)) {
> +        const char *err = ret < 0 ? strerror(-ret) : "short read";
> +
> +        error_setg(errp, "failed to read PCI MSIX PBA: %s", err);
>          return false;
>      }
>  
> @@ -1741,10 +1781,12 @@ static void vfio_bar_prepare(VFIOPCIDevice *vdev, int nr)
>      }
>  
>      /* Determine what type of BAR this is for registration */
> -    ret = pread(vdev->vbasedev.fd, &pci_bar, sizeof(pci_bar),
> -                vdev->config_offset + PCI_BASE_ADDRESS_0 + (4 * nr));
> +    ret = vfio_pci_config_space_read(vdev, PCI_BASE_ADDRESS_0 + (4 * nr),
> +                                     sizeof(pci_bar), &pci_bar);
>      if (ret != sizeof(pci_bar)) {
> -        error_report("vfio: Failed to read BAR %d (%m)", nr);
> +        const char *err =  ret < 0 ? strerror(-ret) : "short read";
> +
> +        error_report("vfio: Failed to read BAR %d: %s", nr, err);
>          return;
>      }
>  
> @@ -2448,21 +2490,25 @@ void vfio_pci_pre_reset(VFIOPCIDevice *vdev)
>  
>  void vfio_pci_post_reset(VFIOPCIDevice *vdev)
>  {
> +    VFIODevice *vbasedev = &vdev->vbasedev;
>      Error *err = NULL;
> -    int nr;
> +    int ret, nr;
>  
>      if (!vfio_intx_enable(vdev, &err)) {
>          error_reportf_err(err, VFIO_MSG_PREFIX, vdev->vbasedev.name);
>      }
>  
>      for (nr = 0; nr < PCI_NUM_REGIONS - 1; ++nr) {
> -        off_t addr = vdev->config_offset + PCI_BASE_ADDRESS_0 + (4 * nr);
> +        off_t addr = PCI_BASE_ADDRESS_0 + (4 * nr);
>          uint32_t val = 0;
>          uint32_t len = sizeof(val);
>  
> -        if (pwrite(vdev->vbasedev.fd, &val, len, addr) != len) {
> -            error_report("%s(%s) reset bar %d failed: %m", __func__,
> -                         vdev->vbasedev.name, nr);
> +        ret = vfio_pci_config_space_write(vdev, addr, len, &val);
> +        if (ret != len) {
> +            const char *errmsg = ret < 0 ? strerror(-ret) : "short write";
> +
> +            error_report("%s(%s) reset bar %d failed: %s", __func__,
> +                         vbasedev->name, nr, errmsg);
>          }
>      }
>  
> @@ -3099,6 +3145,7 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
>      int i, ret;
>      char uuid[UUID_STR_LEN];
>      g_autofree char *name = NULL;
> +    size_t config_space_size;
>  
>      if (vbasedev->fd < 0 && !vbasedev->sysfsdev) {
>          if (!(~vdev->host.domain || ~vdev->host.bus ||
> @@ -3153,13 +3200,14 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
>          goto error;
>      }
>  
> +    config_space_size = MIN(pci_config_size(&vdev->pdev), vdev->config_size);
> +
>      /* Get a copy of config space */
> -    ret = pread(vbasedev->fd, vdev->pdev.config,
> -                MIN(pci_config_size(&vdev->pdev), vdev->config_size),
> -                vdev->config_offset);
> -    if (ret < (int)MIN(pci_config_size(&vdev->pdev), vdev->config_size)) {
> -        ret = ret < 0 ? -errno : -EFAULT;
> -        error_setg_errno(errp, -ret, "failed to read device config space");
> +    ret = vfio_pci_config_space_read(vdev, 0, config_space_size,
> +                                     vdev->pdev.config);
> +    if (ret < (int)config_space_size) {
> +        ret = ret < 0 ? -ret : EFAULT;
> +        error_setg_errno(errp, ret, "failed to read device config space");
>          goto error;
>      }
>