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[82.30.61.225]) by smtp.gmail.com with ESMTPSA id h17sm229723wrx.55.2021.10.07.10.22.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 07 Oct 2021 10:22:36 -0700 (PDT) Date: Thu, 7 Oct 2021 18:22:34 +0100 From: "Dr. David Alan Gilbert" To: Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= Cc: Richard Henderson , Eric Blake , "Daniel P . Berrange" , qemu-devel@nongnu.org, Paolo Bonzini , Eduardo Habkost , kvm@vger.kernel.org, "Michael S. Tsirkin" , James Bottomley , Brijesh Singh , Sergio Lopez , Dov Murik Subject: Re: [PATCH v4 16/23] target/i386/sev: Remove stubs by using code elision Message-ID: References: <20211007161716.453984-1-philmd@redhat.com> <20211007161716.453984-17-philmd@redhat.com> <6080fa16-66aa-570e-93c8-09be2ced9431@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6080fa16-66aa-570e-93c8-09be2ced9431@redhat.com> User-Agent: Mutt/2.0.7 (2021-05-04) Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org * 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 > > > > 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 > >> - * > >> - * 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