[Xen-devel] Re: how to handle paged hypercall args?

[Olaf Hering <olaf@xxxxxxxxx>]

On Wed, Nov 10, Olaf Hering wrote:

> I'm currently reading through the callers of __hvm_copy(). Some of them
> detect HVMCOPY_gfn_paged_out and so some sort of retry. Others just
> ignore the return codes, or turn them into generic errors. In the case
> of copy_from/to_guest each caller needs an audit if a retry is possible.

A frist patch which avoids the BUG_ON is below. 
It turned out that the guests pagetable were just nominated and
paged-out during the preempted do_memory_op hypercall. So
copy_from_guest failed in decrease_reservation() and do_memory_op().

I have added also some error handling in case the copy_to_user fails.
However, only the decrease_reservation() code path is runtime tested and
in fact this whole patch is not yet compile-tested. Its just a heads-up.


So is that an acceptable way to deal with the HVMCOPY_gfn_paged_out
return codes from __hvm_copy?
Or should I explore some different way, like spinning there and possible
let other threads-of-execution make progress while waiting for the gfns
to come back?


Olaf

---
 xen/arch/x86/hvm/hvm.c |    4 ++++
 xen/common/memory.c    |   43 ++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 42 insertions(+), 5 deletions(-)

--- xen-4.0.1-testing.orig/xen/arch/x86/hvm/hvm.c
+++ xen-4.0.1-testing/xen/arch/x86/hvm/hvm.c
@@ -1853,6 +1853,8 @@ unsigned long copy_to_user_hvm(void *to,
 
     rc = hvm_copy_to_guest_virt_nofault((unsigned long)to, (void *)from,
                                         len, 0);
+    if ( rc == HVMCOPY_gfn_paged_out )
+        return -EAGAIN;
     return rc ? len : 0; /* fake a copy_to_user() return code */
 }
 
@@ -1869,6 +1871,8 @@ unsigned long copy_from_user_hvm(void *t
 #endif
 
     rc = hvm_copy_from_guest_virt_nofault(to, (unsigned long)from, len, 0);
+    if ( rc == HVMCOPY_gfn_paged_out )
+        return -EAGAIN;
     return rc ? len : 0; /* fake a copy_from_user() return code */
 }
 
--- xen-4.0.1-testing.orig/xen/common/memory.c
+++ xen-4.0.1-testing/xen/common/memory.c
@@ -47,6 +47,7 @@ static void increase_reservation(struct
 {
     struct page_info *page;
     unsigned long i;
+    unsigned long ctg_ret;
     xen_pfn_t mfn;
     struct domain *d = a->domain;
 
@@ -80,8 +81,14 @@ static void increase_reservation(struct
         if ( !guest_handle_is_null(a->extent_list) )
         {
             mfn = page_to_mfn(page);
-            if ( unlikely(__copy_to_guest_offset(a->extent_list, i, &mfn, 1)) )
+            ctg_ret = __copy_to_guest_offset(a->extent_list, i, &mfn, 1);
+            if ( unlikely(ctg_ret) )
+            {
+                free_domheap_pages(page, a->extent_order);
+                if ( (long)ctg_ret == -EAGAIN )
+                    a->preempted = 1;
                 goto out;
+            }
         }
     }
 
@@ -93,6 +100,7 @@ static void populate_physmap(struct memo
 {
     struct page_info *page;
     unsigned long i, j;
+    unsigned long ctg_ret;
     xen_pfn_t gpfn, mfn;
     struct domain *d = a->domain;
 
@@ -111,8 +119,13 @@ static void populate_physmap(struct memo
             goto out;
         }
 
-        if ( unlikely(__copy_from_guest_offset(&gpfn, a->extent_list, i, 1)) )
+        j = __copy_from_guest_offset(&gpfn, a->extent_list, i, 1);
+        if ( unlikely(j) )
+        {
+            if ( (long)j == -EAGAIN )
+                a->preempted = 1;
             goto out;
+        }
 
         if ( a->memflags & MEMF_populate_on_demand )
         {
@@ -142,8 +155,17 @@ static void populate_physmap(struct memo
                     set_gpfn_from_mfn(mfn + j, gpfn + j);
 
                 /* Inform the domain of the new page's machine address. */ 
-                if ( unlikely(__copy_to_guest_offset(a->extent_list, i, &mfn, 
1)) )
+                ctg_ret = __copy_to_guest_offset(a->extent_list, i, &mfn, 1);
+                if ( unlikely(ctg_ret) )
+                {
+                    for ( j = 0; j < (1 << a->extent_order); j++ )
+                        set_gpfn_from_mfn(mfn + j, INVALID_P2M_ENTRY);
+                    guest_physmap_remove_page(d, gpfn, mfn, a->extent_order);
+                    free_domheap_pages(page, a->extent_order);
+                    if ( (long)ctg_ret == -EAGAIN )
+                        a->preempted = 1;
                     goto out;
+                }
             }
         }
     }
@@ -226,8 +248,13 @@ static void decrease_reservation(struct
             goto out;
         }
 
-        if ( unlikely(__copy_from_guest_offset(&gmfn, a->extent_list, i, 1)) )
+        j = __copy_from_guest_offset(&gmfn, a->extent_list, i, 1);
+        if ( unlikely(j) )
+        {
+            if ( (long)j == -EAGAIN )
+                a->preempted = 1;
             goto out;
+        }
 
         if ( tb_init_done )
         {
@@ -511,6 +538,7 @@ long do_memory_op(unsigned long cmd, XEN
     int rc, op;
     unsigned int address_bits;
     unsigned long start_extent;
+    unsigned long cfg_ret;
     struct xen_memory_reservation reservation;
     struct memop_args args;
     domid_t domid;
@@ -524,8 +552,13 @@ long do_memory_op(unsigned long cmd, XEN
     case XENMEM_populate_physmap:
         start_extent = cmd >> MEMOP_EXTENT_SHIFT;
 
-        if ( copy_from_guest(&reservation, arg, 1) )
+        cfg_ret = copy_from_guest(&reservation, arg, 1);
+        if ( unlikely(cfg_ret) )
+        {
+            if ( (long)cfg_ret == -EAGAIN )
+                return hypercall_create_continuation(__HYPERVISOR_memory_op, 
"lh", cmd, arg);
             return start_extent;
+        }
 
         /* Is size too large for us to encode a continuation? */
         if ( reservation.nr_extents > (ULONG_MAX >> MEMOP_EXTENT_SHIFT) )

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