[xen staging] x86/pv-shim: correct ballooning down for compat guests

[patchbot@xxxxxxx]

commit 1d7fbc535d1d37bdc2cc53ede360b0f6651f7de1
Author:     Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
AuthorDate: Fri Oct 28 15:49:33 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Oct 28 15:49:33 2022 +0200

    x86/pv-shim: correct ballooning down for compat guests
    
    From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
    
    The compat layer for multi-extent memory ops may need to split incoming
    requests. Since the guest handles in the interface structures may not be
    altered, it does so by leveraging do_memory_op()'s continuation
    handling: It hands on non-initial requests with a non-zero start extent,
    with the (native) handle suitably adjusted down. As a result
    do_memory_op() sees only the first of potentially several requests with
    start extent being zero. In order to be usable as overall result, the
    function accumulates args.nr_done, i.e. it initialized the field with
    the start extent. Therefore non-initial requests resulting from the
    split would pass too large a number into pv_shim_offline_memory().
    
    Address that breakage by always calling pv_shim_offline_memory()
    regardless of current hypercall preemption status, with a suitably
    adjusted first argument. Note that this is correct also for the native
    guest case: We now simply "commit" what was completed right away, rather
    than at the end of a series of preemption/re-start cycles. In fact this
    improves overall preemption behavior: There's no longer a potentially
    big chunk of work done non-preemptively at the end of the last
    "iteration".
    
    Fixes: b2245acc60c3 ("xen/pvshim: memory hotplug")
    Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Release-acked-by: Henry Wang <Henry.Wang@xxxxxxx>
---
 xen/common/memory.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/xen/common/memory.c b/xen/common/memory.c
index ae8163a738..a15e5580f3 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1461,22 +1461,17 @@ long do_memory_op(unsigned long cmd, 
XEN_GUEST_HANDLE_PARAM(void) arg)
 
         rc = args.nr_done;
 
-        if ( args.preempted )
-            return hypercall_create_continuation(
-                __HYPERVISOR_memory_op, "lh",
-                op | (rc << MEMOP_EXTENT_SHIFT), arg);
-
 #ifdef CONFIG_X86
         if ( pv_shim && op == XENMEM_decrease_reservation )
-            /*
-             * Only call pv_shim_offline_memory when the hypercall has
-             * finished. Note that nr_done is used to cope in case the
-             * hypercall has failed and only part of the extents where
-             * processed.
-             */
-            pv_shim_offline_memory(args.nr_done, args.extent_order);
+            pv_shim_offline_memory(args.nr_done - start_extent,
+                                   args.extent_order);
 #endif
 
+        if ( args.preempted )
+           return hypercall_create_continuation(
+                __HYPERVISOR_memory_op, "lh",
+                op | (rc << MEMOP_EXTENT_SHIFT), arg);
+
         break;
 
     case XENMEM_exchange:
--
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