Re: [PATCH v4 16/23] target/i386/sev: Remove stubs by using code elision

[Philippe Mathieu-Daudé]

On 10/7/21 19:07, Dr. David Alan Gilbert wrote:
> * Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
>> Only declare sev_enabled() and sev_es_enabled() when CONFIG_SEV is
>> set, to allow the compiler to elide unused code. Remove unnecessary
>> stubs.
>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> 
> What makes it allowed to *rely* on the compiler eliding calls?

I am not aware of a particular requirement on the compiler for code
elision, however we already use this syntax:

$ git grep -A4 'ifdef CONFIG_' include/sysemu/
...
include/sysemu/tcg.h:11:#ifdef CONFIG_TCG
include/sysemu/tcg.h-12-extern bool tcg_allowed;
include/sysemu/tcg.h-13-#define tcg_enabled() (tcg_allowed)
include/sysemu/tcg.h-14-#else
include/sysemu/tcg.h-15-#define tcg_enabled() 0
...

Cc'ing Richard/Eric/Daniel who have more experience with compiler
features in case they can enlighten me here.

>> ---
>>  target/i386/sev.h       | 14 ++++++++++++--
>>  target/i386/cpu.c       | 13 +++++++------
>>  target/i386/sev-stub.c  | 41 -----------------------------------------
>>  target/i386/meson.build |  2 +-
>>  4 files changed, 20 insertions(+), 50 deletions(-)
>>  delete mode 100644 target/i386/sev-stub.c
>>
>> diff --git a/target/i386/sev.h b/target/i386/sev.h
>> index c96072bf78d..d9548e3e642 100644
>> --- a/target/i386/sev.h
>> +++ b/target/i386/sev.h
>> @@ -14,6 +14,10 @@
>>  #ifndef QEMU_SEV_I386_H
>>  #define QEMU_SEV_I386_H
>>  
>> +#ifndef CONFIG_USER_ONLY
>> +#include CONFIG_DEVICES /* CONFIG_SEV */
>> +#endif
>> +
>>  #include "exec/confidential-guest-support.h"
>>  #include "qapi/qapi-types-misc-target.h"
>>  
>> @@ -35,8 +39,14 @@ typedef struct SevKernelLoaderContext {
>>      size_t cmdline_size;
>>  } SevKernelLoaderContext;
>>  
>> -bool sev_enabled(void);
>> -extern bool sev_es_enabled(void);
>> +#ifdef CONFIG_SEV
>> + bool sev_enabled(void);
>> +bool sev_es_enabled(void);
>> +#else
>> +#define sev_enabled() 0
>> +#define sev_es_enabled() 0
>> +#endif
>> +
>>  extern SevInfo *sev_get_info(void);
>>  extern uint32_t sev_get_cbit_position(void);
>>  extern uint32_t sev_get_reduced_phys_bits(void);
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index 8289dc87bd5..fc3ed80ef1e 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -5764,12 +5764,13 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
>> uint32_t count,
>>          *edx = 0;
>>          break;
>>      case 0x8000001F:
>> -        *eax = sev_enabled() ? 0x2 : 0;
>> -        *eax |= sev_es_enabled() ? 0x8 : 0;
>> -        *ebx = sev_get_cbit_position();
>> -        *ebx |= sev_get_reduced_phys_bits() << 6;
>> -        *ecx = 0;
>> -        *edx = 0;
>> +        *eax = *ebx = *ecx = *edx = 0;
>> +        if (sev_enabled()) {
>> +            *eax = 0x2;
>> +            *eax |= sev_es_enabled() ? 0x8 : 0;
>> +            *ebx = sev_get_cbit_position();
>> +            *ebx |= sev_get_reduced_phys_bits() << 6;
>> +        }
>>          break;
>>      default:
>>          /* reserved values: zero */
>> diff --git a/target/i386/sev-stub.c b/target/i386/sev-stub.c
>> deleted file mode 100644
>> index 7e8b6f9a259..00000000000
>> --- a/target/i386/sev-stub.c
>> +++ /dev/null
>> @@ -1,41 +0,0 @@
>> -/*
>> - * QEMU SEV stub
>> - *
>> - * Copyright Advanced Micro Devices 2018
>> - *
>> - * Authors:
>> - *      Brijesh Singh <brijesh.singh@amd.com>
>> - *
>> - * This work is licensed under the terms of the GNU GPL, version 2 or later.
>> - * See the COPYING file in the top-level directory.
>> - *
>> - */
>> -
>> -#include "qemu/osdep.h"
>> -#include "qapi/error.h"
>> -#include "sev.h"
>> -
>> -bool sev_enabled(void)
>> -{
>> -    return false;
>> -}
>> -
>> -uint32_t sev_get_cbit_position(void)
>> -{
>> -    return 0;
>> -}
>> -
>> -uint32_t sev_get_reduced_phys_bits(void)
>> -{
>> -    return 0;
>> -}
>> -
>> -bool sev_es_enabled(void)
>> -{
>> -    return false;
>> -}
>> -
>> -bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
>> -{
>> -    g_assert_not_reached();
>> -}
>> diff --git a/target/i386/meson.build b/target/i386/meson.build
>> index a4f45c3ec1d..ae38dc95635 100644
>> --- a/target/i386/meson.build
>> +++ b/target/i386/meson.build
>> @@ -6,7 +6,7 @@
>>    'xsave_helper.c',
>>    'cpu-dump.c',
>>  ))
>> -i386_ss.add(when: 'CONFIG_SEV', if_true: files('host-cpu.c'), if_false: 
>> files('sev-stub.c'))
>> +i386_ss.add(when: 'CONFIG_SEV', if_true: files('host-cpu.c'))
>>  
>>  # x86 cpu type
>>  i386_ss.add(when: 'CONFIG_KVM', if_true: files('host-cpu.c'))
>> -- 
>> 2.31.1
>>