[PATCH] tools/xg: increase LZMA_BLOCK_SIZE for uncompressing the kernel

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Linux 6.12-rc2 fails to decompress with the current 128MiB, contrary to
the code comment. It results in a failure like this:

    domainbuilder: detail: xc_dom_kernel_file: 
filename="/var/lib/qubes/vm-kernels/6.12-rc2-1.1.fc37/vmlinuz"
    domainbuilder: detail: xc_dom_malloc_filemap    : 12104 kB
    domainbuilder: detail: xc_dom_module_file: 
filename="/var/lib/qubes/vm-kernels/6.12-rc2-1.1.fc37/initramfs"
    domainbuilder: detail: xc_dom_malloc_filemap    : 7711 kB
    domainbuilder: detail: xc_dom_boot_xen_init: ver 4.19, caps xen-3.0-x86_64 
hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
    domainbuilder: detail: xc_dom_parse_image: called
    domainbuilder: detail: xc_dom_find_loader: trying multiboot-binary loader 
...
    domainbuilder: detail: loader probe failed
    domainbuilder: detail: xc_dom_find_loader: trying HVM-generic loader ...
    domainbuilder: detail: loader probe failed
    domainbuilder: detail: xc_dom_find_loader: trying Linux bzImage loader ...
    domainbuilder: detail: _xc_try_lzma_decode: XZ decompression error: Memory 
usage limit reached
    xc: error: panic: xg_dom_bzimageloader.c:761: xc_dom_probe_bzimage_kernel 
unable to XZ decompress kernel: Invalid kernel
    domainbuilder: detail: loader probe failed
    domainbuilder: detail: xc_dom_find_loader: trying ELF-generic loader ...
    domainbuilder: detail: loader probe failed
    xc: error: panic: xg_dom_core.c:689: xc_dom_find_loader: no loader found: 
Invalid kernel
    libxl: error: libxl_dom.c:566:libxl__build_dom: xc_dom_parse_image failed

The important part: XZ decompression error: Memory usage limit reached

This looks to be related to the following change in Linux:
8653c909922743bceb4800e5cc26087208c9e0e6 ("xz: use 128 MiB dictionary and force 
single-threaded mode")

Fix this by increasing the block size to 256MiB. And remove the
misleading comment (from lack of better ideas).

Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
---
 tools/libs/guest/xg_dom_bzimageloader.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/tools/libs/guest/xg_dom_bzimageloader.c 
b/tools/libs/guest/xg_dom_bzimageloader.c
index c6ee6d83e7c6..1fb4e5a1f728 100644
--- a/tools/libs/guest/xg_dom_bzimageloader.c
+++ b/tools/libs/guest/xg_dom_bzimageloader.c
@@ -272,8 +272,7 @@ static int _xc_try_lzma_decode(
     return retval;
 }
 
-/* 128 Mb is the minimum size (half-way) documented to work for all inputs. */
-#define LZMA_BLOCK_SIZE (128*1024*1024)
+#define LZMA_BLOCK_SIZE (256*1024*1024)
 
 static int xc_try_xz_decode(
     struct xc_dom_image *dom, void **blob, size_t *size)
-- 
2.46.0