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:
> 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

So that I'm fine with, the bit I'm more worried about is the bit where
inside the if () we call functions (like sev_get_cbit_position )  which
we know the compiler will elide; I'm sure any sane compiler will,
but.....

Looking at your example, in cpu.c there's still places that ifdef around
areas with tcg_enabled.

Dave

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