Re: [PATCH] target/i386: svm: fix sign extension of exit code

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/17/25 10:42, Richard Henderson wrote:
> On 11/15/25 01:26, Paolo Bonzini wrote:
>> -void cpu_vmexit(CPUX86State *env, uint32_t exit_code, uint64_t 
>> exit_info_1,
>> +void cpu_vmexit(CPUX86State *env, uint64_t exit_code, uint64_t 
>> exit_info_1,
>>                   uintptr_t retaddr)
>>   {
>>       CPUState *cs = env_cpu(env);
>> @@ -732,7 +732,7 @@ void cpu_vmexit(CPUX86State *env, uint32_t 
>> exit_code, uint64_t exit_info_1,
>>       qemu_log_mask(CPU_LOG_TB_IN_ASM, "vmexit(%08x, %016" PRIx64 ", 
>> %016"
>>                     PRIx64 ", " TARGET_FMT_lx ")!\n",
>> -                  exit_code, exit_info_1,
>> +                  (uint32_t)exit_code, exit_info_1,
> 
> Why cast instead of printing all 64 bits?

Because in practice exit_code is either a very small negative value 
(-1...-4) or a positive value.  For QEMU in addition the positive value 
will also be small (less than 16 bits); values between 0x8000_0000 and 
0xffff_ffff could happen in principle but are for use by software and by 
the processor[1].  So the high 32 bits are basically unused, and the 
cast removes eight zeroes or f's from the log.

Paolo

[1] see for example arch/x86/include/uapi/asm/svm.h in Linux:

#define SVM_VMGEXIT_MMIO_READ                   0x80000001
#define SVM_VMGEXIT_MMIO_WRITE                  0x80000002
#define SVM_VMGEXIT_NMI_COMPLETE                0x80000003
#define SVM_VMGEXIT_AP_HLT_LOOP                 0x80000004
#define SVM_VMGEXIT_AP_JUMP_TABLE               0x80000005
#define SVM_VMGEXIT_PSC                         0x80000010
#define SVM_VMGEXIT_GUEST_REQUEST               0x80000011
#define SVM_VMGEXIT_EXT_GUEST_REQUEST           0x80000012
#define SVM_VMGEXIT_AP_CREATION                 0x80000013
#define SVM_VMGEXIT_SNP_RUN_VMPL                0x80000018
#define SVM_VMGEXIT_SAVIC                       0x8000001a
#define SVM_VMGEXIT_HV_FEATURES                 0x8000fffd
#define SVM_VMGEXIT_TERM_REQUEST                0x8000fffe
#define SVM_VMGEXIT_UNSUPPORTED_EVENT           0x8000ffff
#define SVM_EXIT_SW                             0xf0000000

SVM_VMGEXIT_* values are actually stored by the guest in its own memory, 
not by the processor; whereas SVM_EXIT_SW is only used in nested 
virtualization scenarios and will not be passed to cpu_vmexit.