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Re: [PATCH v2] libelf: improve PVH elfnote parsing

On 20.05.21 11:28, Jan Beulich wrote:
On 20.05.2021 11:27, Roger Pau Monné wrote:
On Wed, May 19, 2021 at 12:34:19PM +0200, Jan Beulich wrote:
On 18.05.2021 16:47, Roger Pau Monne wrote:
@@ -425,8 +425,11 @@ static elf_errorstatus elf_xen_addr_calc_check(struct 
elf_binary *elf,
          return -1;
- /* Initial guess for virt_base is 0 if it is not explicitly defined. */
-    if ( parms->virt_base == UNSET_ADDR )
+    /*
+     * Initial guess for virt_base is 0 if it is not explicitly defined in the
+     * PV case. For PVH virt_base is forced to 0 because paging is disabled.
+     */
+    if ( parms->virt_base == UNSET_ADDR || hvm )
          parms->virt_base = 0;
          elf_msg(elf, "ELF: VIRT_BASE unset, using %#" PRIx64 "\n",

This message is wrong now if hvm is true but parms->virt_base != UNSET_ADDR.
Best perhaps is to avoid emitting the message altogether when hvm is true.
(Since you'll be touching it anyway, perhaps a good opportunity to do
with passing parms->virt_base to elf_msg(), and instead simply use a literal

@@ -441,8 +444,10 @@ static elf_errorstatus elf_xen_addr_calc_check(struct 
elf_binary *elf,
       * If we are using the modern ELF notes interface then the default
       * is 0.
+     *
+ * For PVH this is forced to 0, as it's already a legacy option
for PV.
-    if ( parms->elf_paddr_offset == UNSET_ADDR )
+    if ( parms->elf_paddr_offset == UNSET_ADDR || hvm )
          if ( parms->elf_note_start )

Don't you want "|| hvm" here as well, or alternatively suppress the
fallback to the __xen_guest section in the PVH case (near the end of

The legacy __xen_guest section doesn't support PHYS32_ENTRY, so yes,
that part could be completely skipped when called from an HVM

I think I will fix that in another patch though if you are OK, as
it's not strictly related to the calculation fixes done here.

That's fine; it wants to be a prereq to the one here then, though,
I think.

Would it be possible to add some comment to xen/include/public/elfnote.h
Indicating which elfnotes are evaluated for which guest types, including
a hint which elfnotes _have_ been evaluated before this series? This
will help cleaning up guests regarding advertisement of elfnotes
(something I've been planning to do for the Linux kernel).


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