target/riscv/kvm.c: fix mvendorid size in vcpu_set_machine_ids()

cpu->cfg.mvendorid is a 32 bit field and kvm_set_one_reg() always write
a target_ulong val, i.e. a 64 bit field in a 64 bit host.

Given that we're passing a pointer to the mvendorid field, the reg is
reading 64 bits starting from mvendorid and going 32 bits in the next
field, marchid. Here's an example:

$ ./qemu-system-riscv64 -machine virt,accel=kvm -m 2G -smp 1 \
   -cpu rv64,marchid=0xab,mvendorid=0xcd,mimpid=0xef(...)

(inside the guest)
 # cat /proc/cpuinfo
processor	: 0
hart		: 0
isa		: rv64imafdc_zicbom_zicboz_zihintpause_zbb_sstc
mmu		: sv57
mvendorid	: 0xab000000cd
marchid		: 0xab
mimpid		: 0xef

'mvendorid' was written as a combination of 0xab (the value from the
adjacent field, marchid) and its intended value 0xcd.

Fix it by assigning cpu->cfg.mvendorid to a target_ulong var 'reg' and
use it as input for kvm_set_one_reg(). Here's the result with this patch
applied and using the same QEMU command line:

 # cat /proc/cpuinfo
processor	: 0
hart		: 0
isa		: rv64imafdc_zicbom_zicboz_zihintpause_zbb_sstc
mmu		: sv57
mvendorid	: 0xcd
marchid		: 0xab
mimpid		: 0xef

This bug affects only the generic (rv64) CPUs when running with KVM in a
64 bit env since the 'host' CPU does not allow the machine IDs to be
changed via command line.

Fixes: 1fb5a62 ("target/riscv: handle mvendorid/marchid/mimpid for KVM CPUs")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20230802180058.281385-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/kvm.c

@@ -852,12 +852,19 @@ void kvm_arch_init_irq_routing(KVMState *s)
 static int kvm_vcpu_set_machine_ids(RISCVCPU *cpu, CPUState *cs)
 {
     CPURISCVState *env = &cpu->env;
+    target_ulong reg;
     uint64_t id;
     int ret;
 
     id = kvm_riscv_reg_id(env, KVM_REG_RISCV_CONFIG,
                           KVM_REG_RISCV_CONFIG_REG(mvendorid));
-    ret = kvm_set_one_reg(cs, id, &cpu->cfg.mvendorid);
+    /*
+     * cfg.mvendorid is an uint32 but a target_ulong will
+     * be written. Assign it to a target_ulong var to avoid
+     * writing pieces of other cpu->cfg fields in the reg.
+     */
+    reg = cpu->cfg.mvendorid;
+    ret = kvm_set_one_reg(cs, id, &reg);
     if (ret != 0) {
         return ret;
     }