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

[Dr. David Alan Gilbert]

* 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?

Dave

> ---
>  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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK