i386/xen: implement HYPERVISOR_xen_version

This is just meant to serve as an example on how we can implement
hypercalls. xen_version specifically since Qemu does all kind of
feature controllability. So handling that here seems appropriate.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
[dwmw2: Implement kvm_gva_rw() safely]
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

target/i386/kvm/xen-emu.c

@@ -14,9 +14,55 @@
 #include "sysemu/kvm_int.h"
 #include "sysemu/kvm_xen.h"
 #include "kvm/kvm_i386.h"
+#include "exec/address-spaces.h"
 #include "xen-emu.h"
 #include "trace.h"
 
+#include "hw/xen/interface/version.h"
+
+static int kvm_gva_rw(CPUState *cs, uint64_t gva, void *_buf, size_t sz,
+                      bool is_write)
+{
+    uint8_t *buf = (uint8_t *)_buf;
+    int ret;
+
+    while (sz) {
+        struct kvm_translation tr = {
+            .linear_address = gva,
+        };
+
+        size_t len = TARGET_PAGE_SIZE - (tr.linear_address & ~TARGET_PAGE_MASK);
+        if (len > sz) {
+            len = sz;
+        }
+
+        ret = kvm_vcpu_ioctl(cs, KVM_TRANSLATE, &tr);
+        if (ret || !tr.valid || (is_write && !tr.writeable)) {
+            return -EFAULT;
+        }
+
+        cpu_physical_memory_rw(tr.physical_address, buf, len, is_write);
+
+        buf += len;
+        sz -= len;
+        gva += len;
+    }
+
+    return 0;
+}
+
+static inline int kvm_copy_from_gva(CPUState *cs, uint64_t gva, void *buf,
+                                    size_t sz)
+{
+    return kvm_gva_rw(cs, gva, buf, sz, false);
+}
+
+static inline int kvm_copy_to_gva(CPUState *cs, uint64_t gva, void *buf,
+                                  size_t sz)
+{
+    return kvm_gva_rw(cs, gva, buf, sz, true);
+}
+
 int kvm_xen_init(KVMState *s, uint32_t hypercall_msr)
 {
     const int required_caps = KVM_XEN_HVM_CONFIG_HYPERCALL_MSR |
@@ -87,6 +133,43 @@ uint32_t kvm_xen_get_caps(void)
     return kvm_state->xen_caps;
 }
 
+static bool kvm_xen_hcall_xen_version(struct kvm_xen_exit *exit, X86CPU *cpu,
+                                     int cmd, uint64_t arg)
+{
+    int err = 0;
+
+    switch (cmd) {
+    case XENVER_get_features: {
+        struct xen_feature_info fi;
+
+        /* No need for 32/64 compat handling */
+        qemu_build_assert(sizeof(fi) == 8);
+
+        err = kvm_copy_from_gva(CPU(cpu), arg, &fi, sizeof(fi));
+        if (err) {
+            break;
+        }
+
+        fi.submap = 0;
+        if (fi.submap_idx == 0) {
+            fi.submap |= 1 << XENFEAT_writable_page_tables |
+                         1 << XENFEAT_writable_descriptor_tables |
+                         1 << XENFEAT_auto_translated_physmap |
+                         1 << XENFEAT_supervisor_mode_kernel;
+        }
+
+        err = kvm_copy_to_gva(CPU(cpu), arg, &fi, sizeof(fi));
+        break;
+    }
+
+    default:
+        return false;
+    }
+
+    exit->u.hcall.result = err;
+    return true;
+}
+
 static bool do_kvm_xen_handle_exit(X86CPU *cpu, struct kvm_xen_exit *exit)
 {
     uint16_t code = exit->u.hcall.input;
@@ -97,6 +180,9 @@ static bool do_kvm_xen_handle_exit(X86CPU *cpu, struct kvm_xen_exit *exit)
     }
 
     switch (code) {
+    case __HYPERVISOR_xen_version:
+        return kvm_xen_hcall_xen_version(exit, cpu, exit->u.hcall.params[0],
+                                         exit->u.hcall.params[1]);
     default:
         return false;
     }