[PATCH for-4.9 2/4] tools/libxc: Avoid generating inappropriate zero-content records

[Andrew Cooper <andrew.cooper3@citrix.com>]

The code as written attempted to elide zero-content records, as such records
serve no purpose but come with a performance hit.  Unfortunately, in the case
where the hypervisor reported max size is non-zero, but the actual size is
zero, the record is not elided.

This previously tripped up the sanity checks in the restore side of migration,
but as the underlying reasons for eliding the records in the first place are
still valid, fix the elision logic.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Julien Grall <julien.grall@arm.com>
---
 tools/libxc/xc_sr_save_x86_hvm.c |  4 ++++
 tools/libxc/xc_sr_save_x86_pv.c  | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/tools/libxc/xc_sr_save_x86_hvm.c b/tools/libxc/xc_sr_save_x86_hvm.c
index e485928..fc5c6ea 100644
--- a/tools/libxc/xc_sr_save_x86_hvm.c
+++ b/tools/libxc/xc_sr_save_x86_hvm.c
@@ -112,6 +112,10 @@ static int write_hvm_params(struct xc_sr_context *ctx)
         }
     }
 
+    /* No params? Skip this record. */
+    if ( hdr.count == 0 )
+        return 0;
+
     rc = write_split_record(ctx, &rec, entries, hdr.count * sizeof(*entries));
     if ( rc )
         PERROR("Failed to write HVM_PARAMS record");
diff --git a/tools/libxc/xc_sr_save_x86_pv.c b/tools/libxc/xc_sr_save_x86_pv.c
index f218d17..36b1058 100644
--- a/tools/libxc/xc_sr_save_x86_pv.c
+++ b/tools/libxc/xc_sr_save_x86_pv.c
@@ -609,6 +609,10 @@ static int write_one_vcpu_extended(struct xc_sr_context *ctx, uint32_t id)
         return -1;
     }
 
+    /* No content? Skip the record. */
+    if ( domctl.u.ext_vcpucontext.size == 0 )
+        return 0;
+
     return write_split_record(ctx, &rec, &domctl.u.ext_vcpucontext,
                               domctl.u.ext_vcpucontext.size);
 }
@@ -664,6 +668,10 @@ static int write_one_vcpu_xsave(struct xc_sr_context *ctx, uint32_t id)
         goto err;
     }
 
+    /* No xsave state? Skip this record. */
+    if ( domctl.u.vcpuextstate.size == 0 )
+        goto out;
+
     rc = write_split_record(ctx, &rec, buffer, domctl.u.vcpuextstate.size);
     if ( rc )
         goto err;
@@ -730,6 +738,10 @@ static int write_one_vcpu_msrs(struct xc_sr_context *ctx, uint32_t id)
         goto err;
     }
 
+    /* No MSRs? Skip this record. */
+    if ( domctl.u.vcpu_msrs.msr_count == 0 )
+        goto out;
+
     rc = write_split_record(ctx, &rec, buffer,
                             domctl.u.vcpu_msrs.msr_count *
                             sizeof(xen_domctl_vcpu_msr_t));
-- 
2.1.4


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