spapr: nested: Add support for DPDES SPR in GSB for TCG L0

The DPDES support for doorbell emulation and handling for KVM on PAPR
guests was added in Linux via [1]. Subsequently, a new GSB (Guest State
Buffer) element for DPDES was added in Linux; the same has been missing
in TCG L0 implementation. Add support for DPDES register's APIv2 GSB
element and required handling in `spapr_nested.c`.

Currently, booting a KVM guest inside a QEMU TCG guest fails with the
following crash. The crash occurs while handling the GUEST_RUN_VCPU
hcall made in TCG L0. In the hcall handling path, map_and_getset_state()
calls getset_state(), which, in turn, calls guest_state_request_check()
to validate the GSR (Guest State Request) elements. During this process,
guest_state_request_check() iterates over the GSR elements and receives
a NULL return code from guest_state_element_type_find() for the type
variable corresponding to the DPDES register's elemetn ID (which was
unknown to TCG L0). Subsequently, getset_state() returns H_P3,
ultimately leading to the hcall failure and causing the KVM guest to
crash.

  KVM: unknown exit, hardware reason ffffffffffffffea
  [ ... KVM register dump ... ]

Fix this by adding the required support in TCG L0 implementation of
APIv2.

[1] https://lore.kernel.org/all/20240605113913.83715-1-gautam@linux.ibm.com/

Fixes: 4a575f9a0567 ("spapr: nested: Initialize the GSB elements lookup table.")
Suggested-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

hw/ppc/spapr_nested.c

@@ -982,6 +982,7 @@ struct guest_state_element_type guest_state_element_types[] = {
     GUEST_STATE_ELEMENT_ENV_DW(GSB_VCPU_SPR_FSCR,  fscr),
     GUEST_STATE_ELEMENT_ENV_W(GSB_VCPU_SPR_PSPB,   pspb),
     GUEST_STATE_ELEMENT_ENV_DW(GSB_VCPU_SPR_CTRL,  ctrl),
+    GUEST_STATE_ELEMENT_ENV_DW(GSB_VCPU_SPR_DPDES, dpdes),
     GUEST_STATE_ELEMENT_ENV_W(GSB_VCPU_SPR_VRSAVE, vrsave),
     GUEST_STATE_ELEMENT_ENV_DW(GSB_VCPU_SPR_DAR,   dar),
     GUEST_STATE_ELEMENT_ENV_W(GSB_VCPU_SPR_DSISR,  dsisr),

include/hw/ppc/spapr_nested.h

@@ -99,7 +99,8 @@
 #define GSB_VCPU_SPR_HASHKEYR   0x1050
 #define GSB_VCPU_SPR_HASHPKEYR  0x1051
 #define GSB_VCPU_SPR_CTRL       0x1052
-                    /* RESERVED 0x1053 - 0x1FFF */
+#define GSB_VCPU_SPR_DPDES      0x1053
+                    /* RESERVED 0x1054 - 0x1FFF */
 #define GSB_VCPU_SPR_CR         0x2000
 #define GSB_VCPU_SPR_PIDR       0x2001
 #define GSB_VCPU_SPR_DSISR      0x2002