Re: [MINI-OS PATCH 2/2] x86: don't use a memory page for mapping the shared info page

[Jürgen Groß]

On 28.07.25 16:21, Jan Beulich wrote:

On 08.07.2025 08:37, Juergen Gross wrote:

--- a/hypervisor.c
+++ b/hypervisor.c
@@ -27,8 +27,10 @@

  #include <mini-os/lib.h>
+#include <mini-os/e820.h>
  #include <mini-os/hypervisor.h>
  #include <mini-os/events.h>
+#include <mini-os/mm.h>
  #include <xen/memory.h>

@@ -37,7 +39,8 @@ EXPORT_SYMBOL(hypercall_page);
      ((sh)->evtchn_pending[idx] & ~(sh)->evtchn_mask[idx])

-extern shared_info_t shared_info;
+static unsigned long shinfo_pfn;
+static unsigned long shinfo_va;

  {
@@ -69,14 +72,16 @@ shared_info_t *map_shared_info(void)
  {
      struct xen_add_to_physmap xatp;

      xatp.domid = DOMID_SELF;
      xatp.idx = 0;
      xatp.space = XENMAPSPACE_shared_info;
-    xatp.gpfn = virt_to_pfn(&shared_info);
+    xatp.gpfn = shinfo_pfn;
      if ( HYPERVISOR_memory_op(XENMEM_add_to_physmap, &xatp) != 0 )
          BUG();
+    shinfo_va = map_frame_virt(shinfo_pfn);

#include <mini-os/os.h> EXPORT_SYMBOL(hypercall_page); #ifndef CONFIG_PARAVIRT int hvm_get_parameter(int idx, uint64_t *value) + shinfo_pfn = e820_get_reserved_pfns(1);

In the PV variant you first check whether you already have a VA. Why is
that needed there, but not here? (Originally I meant to ask why you don't
use map_frame_virt() there as well.)

Good catch. It is needed here, otherwise we'll leak the virtual page
during a suspend/resume cycle.

Talking of map_frame_virt() - I take it its 2nd parameter being named
"mfn" is kind of stale (pre-dating the addition of PVH support)?

I think it was misnamed back then.

@@ -84,9 +89,11 @@ void unmap_shared_info(void)
      struct xen_remove_from_physmap xrtp;

-    xrtp.gpfn = virt_to_pfn(&shared_info);
+    xrtp.gpfn = shinfo_pfn;
      if ( HYPERVISOR_memory_op(XENMEM_remove_from_physmap, &xrtp) != 0 )
          BUG();
+    unmap_frames(shinfo_va, 1);

xrtp.domid = DOMID_SELF;

Better do this ahead of the hypercall?

Agreed.


Juergen

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