Introdce struct arch_vcpu to hold RISC-V vCPU-specific state.

The structure contains:
  - Guest-visible CSR state, used to save and restore vCPU execution
    state across context switches. hstatus isn't added here as it is
    already part of cpu_user_regs struct.
  - Callee-saved registers used to preserve Xen’s own execution context
    when switching between vCPU stacks.

It is going to be used in the following way (pseudocode):
  context_switch(prev_vcpu, next_vcpu):
    ...

    /* Switch from previous stack to the next stack. */
    __context_switch(prev_vcpu, next_vcpu);

    ...
    schedule_tail(prev_vcpu):
       Save and restore vCPU's CSRs.

The Xen-saved context allows __context_switch() to switch execution
from the previous vCPU’s stack to the next vCPU’s stack and later resume
execution on the original stack when switching back.

During vCPU creation, the Xen-saved context is going to be initialized
with:
  - SP pointing to the newly allocated vCPU stack
  - RA pointing to a helper that performs final vCPU setup before
    transferring control to the guest
After the first execution of __context_switch(), RA naturally points to
the instruction following the call site, and the remaining callee-saved
registers contain the Xen register state at the time of the switch.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
---
Changes in v2:
 - Drop hstatus from struct arch_vcpu as it is stored in struct cpu_user_regs
   which will be stored on top of vCPU's stack.
 - Drop the comment above ra in xen_saved_context struct as it is potentially
   misleading.
---
 xen/arch/riscv/include/asm/domain.h | 57 ++++++++++++++++++++++++++++-
 1 file changed, 55 insertions(+), 2 deletions(-)

diff --git a/xen/arch/riscv/include/asm/domain.h b/xen/arch/riscv/include/asm/domain.h
index 316e7c6c8448..0d9b4c4b525e 100644
--- a/xen/arch/riscv/include/asm/domain.h
+++ b/xen/arch/riscv/include/asm/domain.h
@@ -22,9 +22,62 @@ struct hvm_domain
 struct arch_vcpu_io {
 };
 
-struct arch_vcpu {
+struct arch_vcpu
+{
     struct vcpu_vmid vmid;
-};
+
+    /*
+     * Callee saved registers for Xen's state deep in the callframe used to
+     * switch from prev's stack to the next's stack during context switch.
+     */
+    struct
+    {
+        register_t s0;
+        register_t s1;
+        register_t s2;
+        register_t s3;
+        register_t s4;
+        register_t s5;
+        register_t s6;
+        register_t s7;
+        register_t s8;
+        register_t s9;
+        register_t s10;
+        register_t s11;
+        register_t sp;
+        register_t gp;
+        register_t ra;
+    } xen_saved_context;
+
+    /* CSRs */
+    register_t hedeleg;
+    register_t hideleg;
+    register_t hvip;
+    register_t hip;
+    register_t hie;
+    register_t hgeie;
+    register_t henvcfg;
+    register_t hcounteren;
+    register_t htimedelta;
+    register_t htval;
+    register_t htinst;
+    register_t hstateen0;
+#ifdef CONFIG_RISCV_32
+    register_t henvcfgh;
+    register_t htimedeltah;
+#endif
+
+    /* VCSRs */
+    register_t vsstatus;
+    register_t vsip;
+    register_t vsie;
+    register_t vstvec;
+    register_t vsscratch;
+    register_t vscause;
+    register_t vstval;
+    register_t vsatp;
+    register_t vsepc;
+}  __cacheline_aligned;
 
 struct paging_domain {
     spinlock_t lock;
-- 
2.52.0