Re: [PATCH 3/3] pci: ROM preallocation for incoming migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Apr 25, 2023 at 01:56:03PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On incoming migration we have the following sequence to load option
> ROM:
> 
> 1. On device realize we do normal load ROM from the file
> 
> 2. Than, on incoming migration we rewrite ROM from the incoming RAM
>    block. If sizes mismatch we fail.
> 
> This is not ideal when we migrate to updated distribution: we have to
> keep old ROM files in new distribution and be careful around romfile
> property to load correct ROM file. Which is loaded actually just to
> allocate the ROM with correct length.
> 
> Note, that romsize property doesn't really help: if we try to specify
> it when default romfile is larger, it fails with something like:
> 
> romfile "efi-virtio.rom" (160768 bytes) is too large for ROM size 65536
> 
> This commit brings new behavior for romfile="",romsize=SIZE combination
> of options. Current behavior is just ignore romsize and not load or
> create any ROM.
> 
> Let's instead preallocate ROM, not loading any file. This way we can
> migrate old vm to new environment not thinking about ROM files on
> destination host:
> 
> 1. specify romfile="",romsize=SIZE on target, with correct SIZE
>    (actually, size of romfile on source aligned up to power of two, or
>     just original romsize option on source)
> 
> 2. On device realize we just preallocate ROM, and not load any file
> 
> 3. On incoming migration ROM is filled from the migration stream
> 
> As a bonus we avoid extra reading from ROM file on target.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

why is this a bad idea:
- on source presumably user overrides romfile
- we have a general rule that source and destination flags must match

I propose instead to ignore romfile if qemu is incoming migration
and romsize has been specified.

> ---
>  hw/pci/pci.c | 76 +++++++++++++++++++++++++++++-----------------------
>  1 file changed, 43 insertions(+), 33 deletions(-)
> 
> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> index a442f8fce1..039e762b66 100644
> --- a/hw/pci/pci.c
> +++ b/hw/pci/pci.c
> @@ -2293,17 +2293,21 @@ static void pci_add_option_rom(PCIDevice *pdev, bool is_default_rom,
>  {
>      int64_t size;
>      g_autofree char *path = NULL;
> -    void *ptr;
>      char name[32];
>      const VMStateDescription *vmsd;
> +    bool load_file;
>  
>      if (!pdev->romfile) {
>          return;
>      }
> -    if (strlen(pdev->romfile) == 0) {
> +
> +    load_file = strlen(pdev->romfile) > 0;
> +    if (!load_file && pdev->romsize == -1) {
>          return;
>      }
>  
> +    assert(load_file || !is_default_rom);
> +
>      if (!pdev->rom_bar) {
>          /*
>           * Load rom via fw_cfg instead of creating a rom bar,
> @@ -2329,32 +2333,35 @@ static void pci_add_option_rom(PCIDevice *pdev, bool is_default_rom,
>          return;
>      }
>  
> -    path = qemu_find_file(QEMU_FILE_TYPE_BIOS, pdev->romfile);
> -    if (path == NULL) {
> -        path = g_strdup(pdev->romfile);
> -    }
> +    if (load_file) {
> +        path = qemu_find_file(QEMU_FILE_TYPE_BIOS, pdev->romfile);
> +        if (path == NULL) {
> +            path = g_strdup(pdev->romfile);
> +        }
>  
> -    size = get_image_size(path);
> -    if (size < 0) {
> -        error_setg(errp, "failed to find romfile \"%s\"", pdev->romfile);
> -        return;
> -    } else if (size == 0) {
> -        error_setg(errp, "romfile \"%s\" is empty", pdev->romfile);
> -        return;
> -    } else if (size > 2 * GiB) {
> -        error_setg(errp, "romfile \"%s\" too large (size cannot exceed 2 GiB)",
> -                   pdev->romfile);
> -        return;
> -    }
> -    if (pdev->romsize != -1) {
> -        if (size > pdev->romsize) {
> -            error_setg(errp, "romfile \"%s\" (%u bytes) "
> -                       "is too large for ROM size %u",
> -                       pdev->romfile, (uint32_t)size, pdev->romsize);
> +        size = get_image_size(path);
> +        if (size < 0) {
> +            error_setg(errp, "failed to find romfile \"%s\"", pdev->romfile);
> +            return;
> +        } else if (size == 0) {
> +            error_setg(errp, "romfile \"%s\" is empty", pdev->romfile);
> +            return;
> +        } else if (size > 2 * GiB) {
> +            error_setg(errp,
> +                       "romfile \"%s\" too large (size cannot exceed 2 GiB)",
> +                       pdev->romfile);
>              return;
>          }
> -    } else {
> -        pdev->romsize = pow2ceil(size);
> +        if (pdev->romsize != -1) {
> +            if (size > pdev->romsize) {
> +                error_setg(errp, "romfile \"%s\" (%u bytes) "
> +                           "is too large for ROM size %u",
> +                           pdev->romfile, (uint32_t)size, pdev->romsize);
> +                return;
> +            }
> +        } else {
> +            pdev->romsize = pow2ceil(size);
> +        }
>      }
>  
>      vmsd = qdev_get_vmsd(DEVICE(pdev));
> @@ -2365,15 +2372,18 @@ static void pci_add_option_rom(PCIDevice *pdev, bool is_default_rom,
>      memory_region_init_rom(&pdev->rom, OBJECT(pdev), name, pdev->romsize,
>                             &error_fatal);
>  
> -    ptr = memory_region_get_ram_ptr(&pdev->rom);
> -    if (load_image_size(path, ptr, size) < 0) {
> -        error_setg(errp, "failed to load romfile \"%s\"", pdev->romfile);
> -        return;
> -    }
> +    if (load_file) {
> +        void *ptr = memory_region_get_ram_ptr(&pdev->rom);
>  
> -    if (is_default_rom) {
> -        /* Only the default rom images will be patched (if needed). */
> -        pci_patch_ids(pdev, ptr, size);
> +        if (load_image_size(path, ptr, size) < 0) {
> +            error_setg(errp, "failed to load romfile \"%s\"", pdev->romfile);
> +            return;
> +        }
> +
> +        if (is_default_rom) {
> +            /* Only the default rom images will be patched (if needed). */
> +            pci_patch_ids(pdev, ptr, size);
> +        }
>      }
>  
>      pci_register_bar(pdev, PCI_ROM_SLOT, 0, &pdev->rom);
> -- 
> 2.34.1