On 10/7/21 19:07, Dr. David Alan Gilbert wrote:
> * Philippe Mathieu-Daudé (philmd@xxxxxxxxxx) 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@xxxxxxxxxx>
>
> 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@xxxxxxx>
>> - *
>> - * 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
>>
