[PATCH v2 6/9] tools/libs/guest: add XCFLAGS_COMPRESS_LZ4 to request LZ4 compression

[Marcus Granado <marcus.granado@xxxxxxxxxx>]

Nothing sets ctx->save.compression to XC_SR_COMPRESS_LZ4, so the LZ4
path in write_batch() is unreachable and a caller of xc_domain_save()
has no way to ask for compressed PAGE_DATA records.

Add XCFLAGS_COMPRESS_LZ4 and select LZ4 when it is set.  A caller which
does not set it sees no change; one which does needs a receiver able to
decompress LZ4.

A libxenguest built without a supported liblz4 cannot honour the flag,
so fail up front with EOPNOTSUPP rather than silently send uncompressed
records.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Marcus Granado <marcus.granado@xxxxxxxxxx>
---

Notes:
    Changes in v2:
    - Was v1 5/6.
    - Set ctx->save.compression; the field moved into the save context.
    
    Bit 2 was XCFLAGS_HVM until commit 2004db3ced18 ("tools/save: Drop
    unused parameters from xc_domain_save()") removed it in 4.14.

 tools/include/xenguest.h      |  5 +++--
 tools/libs/guest/xg_sr_save.c | 11 +++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/include/xenguest.h b/tools/include/xenguest.h
index 2a277cb7cd..70301c2f48 100644
--- a/tools/include/xenguest.h
+++ b/tools/include/xenguest.h
@@ -24,8 +24,9 @@
 
 #define XC_NUMA_NO_NODE   (~0U)
 
-#define XCFLAGS_LIVE      (1 << 0)
-#define XCFLAGS_DEBUG     (1 << 1)
+#define XCFLAGS_LIVE         (1 << 0)
+#define XCFLAGS_DEBUG        (1 << 1)
+#define XCFLAGS_COMPRESS_LZ4 (1 << 2) /* LZ4 on save; restorer needs LZ4 */
 
 #define X86_64_B_SIZE   64 
 #define X86_32_B_SIZE   32
diff --git a/tools/libs/guest/xg_sr_save.c b/tools/libs/guest/xg_sr_save.c
index 4fa1d22859..fe3d78bdd6 100644
--- a/tools/libs/guest/xg_sr_save.c
+++ b/tools/libs/guest/xg_sr_save.c
@@ -1166,6 +1166,17 @@ int xc_domain_save(xc_interface *xch, int io_fd, 
uint32_t dom,
     ctx.save.debug = !!(flags & XCFLAGS_DEBUG);
     ctx.save.recv_fd = recv_fd;
 
+    if ( flags & XCFLAGS_COMPRESS_LZ4 )
+    {
+#if defined(XG_SR_LZ4)
+        ctx.save.compression = XC_SR_COMPRESS_LZ4;
+#else
+        ERROR("XCFLAGS_COMPRESS_LZ4 requested but libxenguest was built 
without a supported liblz4");
+        errno = EOPNOTSUPP;
+        return -1;
+#endif
+    }
+
     if ( xc_domain_getinfo_single(xch, dom, &ctx.dominfo) < 0 )
     {
         PERROR("Failed to get domain info");
-- 
2.43.0