Starting with z15 (or newer) we can execute mmio
instructions from userspace. On older platforms
where we don't have these instructions available
we can fallback to using system calls to access
the PCI mapped resources.

This patch adds helper functions for mmio reads
and writes for s390x.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
---
 include/qemu/s390x_pci_mmio.h |  24 ++++++
 util/meson.build              |   2 +
 util/s390x_pci_mmio.c         | 148 ++++++++++++++++++++++++++++++++++
 3 files changed, 174 insertions(+)
 create mode 100644 include/qemu/s390x_pci_mmio.h
 create mode 100644 util/s390x_pci_mmio.c

diff --git a/include/qemu/s390x_pci_mmio.h b/include/qemu/s390x_pci_mmio.h
new file mode 100644
index 0000000000..c5f63ecefa
--- /dev/null
+++ b/include/qemu/s390x_pci_mmio.h
@@ -0,0 +1,24 @@
+/*
+ * s390x PCI MMIO definitions
+ *
+ * Copyright 2025 IBM Corp.
+ * Author(s): Farhan Ali <alifm@linux.ibm.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+#ifndef S390X_PCI_MMIO_H
+#define S390X_PCI_MMIO_H
+
+#ifdef __s390x__
+uint8_t s390x_pci_mmio_read_8(const void *ioaddr);
+uint16_t s390x_pci_mmio_read_16(const void *ioaddr);
+uint32_t s390x_pci_mmio_read_32(const void *ioaddr);
+uint64_t s390x_pci_mmio_read_64(const void *ioaddr);
+
+void s390x_pci_mmio_write_8(void *ioaddr, uint8_t val);
+void s390x_pci_mmio_write_16(void *ioaddr, uint16_t val);
+void s390x_pci_mmio_write_32(void *ioaddr, uint32_t val);
+void s390x_pci_mmio_write_64(void *ioaddr, uint64_t val);
+#endif /* __s390x__ */
+
+#endif /* S390X_PCI_MMIO_H */
diff --git a/util/meson.build b/util/meson.build
index 780b5977a8..acb21592f9 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -131,4 +131,6 @@ elif cpu in ['ppc', 'ppc64']
   util_ss.add(files('cpuinfo-ppc.c'))
 elif cpu in ['riscv32', 'riscv64']
   util_ss.add(files('cpuinfo-riscv.c'))
+elif cpu == 's390x'
+  util_ss.add(files('s390x_pci_mmio.c'))
 endif
diff --git a/util/s390x_pci_mmio.c b/util/s390x_pci_mmio.c
new file mode 100644
index 0000000000..820458a026
--- /dev/null
+++ b/util/s390x_pci_mmio.c
@@ -0,0 +1,148 @@
+/*
+ * s390x PCI MMIO definitions
+ *
+ * Copyright 2025 IBM Corp.
+ * Author(s): Farhan Ali <alifm@linux.ibm.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include <unistd.h>
+#include <sys/syscall.h>
+#include "qemu/s390x_pci_mmio.h"
+#include "elf.h"
+
+union register_pair {
+    unsigned __int128 pair;
+    struct {
+        uint64_t even;
+        uint64_t odd;
+    };
+};
+
+static bool is_mio_supported;
+
+static __attribute__((constructor)) void check_is_mio_supported(void)
+{
+    is_mio_supported = !!(qemu_getauxval(AT_HWCAP) & HWCAP_S390_PCI_MIO);
+}
+
+static uint64_t s390x_pcilgi(const void *ioaddr, size_t len)
+{
+    union register_pair ioaddr_len = { .even = (uint64_t)ioaddr,
+                                       .odd = len };
+    uint64_t val;
+    int cc;
+
+    asm volatile(
+        /* pcilgi */
+        ".insn   rre,0xb9d60000,%[val],%[ioaddr_len]\n"
+        "ipm     %[cc]\n"
+        "srl     %[cc],28\n"
+        : [cc] "=d"(cc), [val] "=d"(val),
+        [ioaddr_len] "+&d"(ioaddr_len.pair) :: "cc");
+
+    if (cc) {
+        val = -1ULL;
+    }
+
+    return val;
+}
+
+static void s390x_pcistgi(void *ioaddr, uint64_t val, size_t len)
+{
+    union register_pair ioaddr_len = {.even = (uint64_t)ioaddr, .odd = len};
+
+    asm volatile (
+        /* pcistgi */
+        ".insn   rre,0xb9d40000,%[val],%[ioaddr_len]\n"
+        : [ioaddr_len] "+&d" (ioaddr_len.pair)
+        : [val] "d" (val)
+        : "cc", "memory");
+}
+
+uint8_t s390x_pci_mmio_read_8(const void *ioaddr)
+{
+    uint8_t val = 0;
+
+    if (is_mio_supported) {
+        val = s390x_pcilgi(ioaddr, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_read, ioaddr, &val, sizeof(val));
+    }
+    return val;
+}
+
+uint16_t s390x_pci_mmio_read_16(const void *ioaddr)
+{
+    uint16_t val = 0;
+
+    if (is_mio_supported) {
+        val = s390x_pcilgi(ioaddr, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_read, ioaddr, &val, sizeof(val));
+    }
+    return val;
+}
+
+uint32_t s390x_pci_mmio_read_32(const void *ioaddr)
+{
+    uint32_t val = 0;
+
+    if (is_mio_supported) {
+        val = s390x_pcilgi(ioaddr, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_read, ioaddr, &val, sizeof(val));
+    }
+    return val;
+}
+
+uint64_t s390x_pci_mmio_read_64(const void *ioaddr)
+{
+    uint64_t val = 0;
+
+    if (is_mio_supported) {
+        val = s390x_pcilgi(ioaddr, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_read, ioaddr, &val, sizeof(val));
+    }
+    return val;
+}
+
+void s390x_pci_mmio_write_8(void *ioaddr, uint8_t val)
+{
+    if (is_mio_supported) {
+        s390x_pcistgi(ioaddr, val, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_write, ioaddr, &val, sizeof(val));
+    }
+}
+
+void s390x_pci_mmio_write_16(void *ioaddr, uint16_t val)
+{
+    if (is_mio_supported) {
+        s390x_pcistgi(ioaddr, val, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_write, ioaddr, &val, sizeof(val));
+    }
+}
+
+void s390x_pci_mmio_write_32(void *ioaddr, uint32_t val)
+{
+    if (is_mio_supported) {
+        s390x_pcistgi(ioaddr, val, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_write, ioaddr, &val, sizeof(val));
+    }
+}
+
+void s390x_pci_mmio_write_64(void *ioaddr, uint64_t val)
+{
+    if (is_mio_supported) {
+        s390x_pcistgi(ioaddr, val, sizeof(val));
+    } else {
+        syscall(__NR_s390_pci_mmio_write, ioaddr, &val, sizeof(val));
+    }
+}
+
-- 
2.43.0