libxl: don't needlessly report "highmem" in use

Commit Message

Jan Beulich Jan. 7, 2020, 2:58 p.m. UTC

Due to the unconditional updating of dom->highmem_end in
libxl__domain_device_construct_rdm() I've observed on a 2Gb HVM guest
with a passed through device (without overly large BARs, and with no RDM
ranges at all)

(d2) RAM in high memory; setting high_mem resource base to 100000000
...
(d2) E820 table:
(d2)  [00]: 00000000:00000000 - 00000000:000a0000: RAM
(d2)  HOLE: 00000000:000a0000 - 00000000:000d0000
(d2)  [01]: 00000000:000d0000 - 00000000:00100000: RESERVED
(d2)  [02]: 00000000:00100000 - 00000000:7f800000: RAM
(d2)  HOLE: 00000000:7f800000 - 00000000:fc000000
(d2)  [03]: 00000000:fc000000 - 00000001:00000000: RESERVED
(d2)  [04]: 00000001:00000000 - 00000001:00000000: RAM

both of which aren't really appropriate in this case. Arrange for this
to not happen.

Signed-off-by: Jan Beulich <[e-mail]>

Comments

Wei Liu Jan. 7, 2020, 4:43 p.m. UTC | #1

On Tue, Jan 07, 2020 at 03:58:07PM +0100, Jan Beulich wrote:
> Due to the unconditional updating of dom->highmem_end in
> libxl__domain_device_construct_rdm() I've observed on a 2Gb HVM guest
> with a passed through device (without overly large BARs, and with no RDM
> ranges at all)
> 
> (d2) RAM in high memory; setting high_mem resource base to 100000000
> ...
> (d2) E820 table:
> (d2)  [00]: 00000000:00000000 - 00000000:000a0000: RAM
> (d2)  HOLE: 00000000:000a0000 - 00000000:000d0000
> (d2)  [01]: 00000000:000d0000 - 00000000:00100000: RESERVED
> (d2)  [02]: 00000000:00100000 - 00000000:7f800000: RAM
> (d2)  HOLE: 00000000:7f800000 - 00000000:fc000000
> (d2)  [03]: 00000000:fc000000 - 00000001:00000000: RESERVED
> (d2)  [04]: 00000001:00000000 - 00000001:00000000: RAM
> 
> both of which aren't really appropriate in this case. Arrange for this
> to not happen.

Indeed. We shouldn't need to move RAM to high address in this
configuration.

Acked-by: Wei Liu <[e-mail]>

Patch

--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -432,7 +432,7 @@  int libxl__domain_device_construct_rdm(l
     uint16_t seg;
     uint8_t bus, devfn;
     uint64_t rdm_start, rdm_size;
-    uint64_t highmem_end = dom->highmem_end ? dom->highmem_end : (1ull<<32);
+    uint64_t highmem_end = dom->highmem_end;
 
     /*
      * We just want to construct RDM once since RDM is specific to the
@@ -557,6 +557,8 @@  int libxl__domain_device_construct_rdm(l
              * We will move downwards lowmem_end so we have to expand
              * highmem_end.
              */
+            if (!highmem_end)
+                highmem_end = 1ull << 32;
             highmem_end += (dom->lowmem_end - rdm_start);
             /* Now move downwards lowmem_end. */
             dom->lowmem_end = rdm_start;
@@ -577,9 +579,10 @@  int libxl__domain_device_construct_rdm(l
         conflict = overlaps_rdm(0, dom->lowmem_end,
                                 rdm_start, rdm_size);
         /* Does this entry conflict with highmem? */
-        conflict |= overlaps_rdm((1ULL<<32),
-                                 dom->highmem_end - (1ULL<<32),
-                                 rdm_start, rdm_size);
+        if (highmem_end)
+            conflict |= overlaps_rdm((1ULL << 32),
+                                     highmem_end - (1ULL << 32),
+                                     rdm_start, rdm_size);
 
         if (!conflict)
             continue;