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Peter Anvin" , Arnd Bergmann , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, Juergen Gross , Nikolay Borisov , Chen Yu , Chen Yu , Qiuxu Zhuo , Prem Nath Dey , Xiaoping Zhou Subject: [PATCH v3] x86/paravirt: Disable virt spinlock on bare metal Date: Tue, 25 Jun 2024 20:54:03 +0800 Message-Id: <20240625125403.187110-1-yu.c.chen@intel.com> X-Mailer: git-send-email 2.25.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The kernel can change spinlock behavior when running as a guest. But this guest-friendly behavior causes performance problems on bare metal. So there's a 'virt_spin_lock_key' static key to switch between the two modes. The static key is always enabled by default (run in guest mode) and should be disabled for bare metal (and in some guests that want native behavior). Performance drop is reported when running encode/decode workload and BenchSEE cache sub-workload. Bisect points to commit ce0a1b608bfc ("x86/paravirt: Silence unused native_pv_lock_init() function warning"). When CONFIG_PARAVIRT_SPINLOCKS is disabled the virt_spin_lock_key is incorrectly set to true on bare metal. The qspinlock degenerates to test-and-set spinlock, which decrease the performance on bare metal. Set the default value of virt_spin_lock_key to false. If booting in a VM, enable this key. Later during the VM initialization, if other high-efficient spinlock is preferred(paravirt-spinlock eg), the virt_spin_lock_key is disabled accordingly. The relation is described as below: X86_FEATURE_HYPERVISOR Y Y Y N CONFIG_PARAVIRT_SPINLOCKS Y Y N Y/N PV spinlock Y N N Y/N virt_spin_lock_key N N Y N Fixes: ce0a1b608bfc ("x86/paravirt: Silence unused native_pv_lock_init() fu= nction warning") Suggested-by: Dave Hansen Suggested-by: Qiuxu Zhuo Suggested-by: Nikolay Borisov Reported-by: Prem Nath Dey Reported-by: Xiaoping Zhou Signed-off-by: Chen Yu Reviewed-by tag from Nikolay? Reviewed-by: Nikolay Borisov --- v2._v3: Change the default value of virt_spin_lock_key from true to false. Enable this key when it is in the VM, and disable it when needed. This makes the code more readable. (Nikolay Borisov) Dropped Reviewed-by because the code has been changed. v1->v2: Refine the commit log per Dave's suggestion. Simplify the fix by directly disabling the virt_spin_lock_key on bare met= al. Collect Reviewed-by from Juergen. --- arch/x86/include/asm/qspinlock.h | 4 ++-- arch/x86/kernel/paravirt.c | 7 +++---- 2 files changed, 5 insertions(+), 6 deletions(-) diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinl= ock.h index a053c1293975..a32bd2aabdf9 100644 --- a/arch/x86/include/asm/qspinlock.h +++ b/arch/x86/include/asm/qspinlock.h @@ -66,13 +66,13 @@ static inline bool vcpu_is_preempted(long cpu) =20 #ifdef CONFIG_PARAVIRT /* - * virt_spin_lock_key - enables (by default) the virt_spin_lock() hijack. + * virt_spin_lock_key - disables (by default) the virt_spin_lock() hijack. * * Native (and PV wanting native due to vCPU pinning) should disable this = key. * It is done in this backwards fashion to only have a single direction ch= ange, * which removes ordering between native_pv_spin_init() and HV setup. */ -DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key); +DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key); =20 /* * Shortcut for the queued_spin_lock_slowpath() function that allows diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c index 5358d43886ad..fec381533555 100644 --- a/arch/x86/kernel/paravirt.c +++ b/arch/x86/kernel/paravirt.c @@ -51,13 +51,12 @@ DEFINE_ASM_FUNC(pv_native_irq_enable, "sti", .noinstr.t= ext); DEFINE_ASM_FUNC(pv_native_read_cr2, "mov %cr2, %rax", .noinstr.text); #endif =20 -DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key); +DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key); =20 void __init native_pv_lock_init(void) { - if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) && - !boot_cpu_has(X86_FEATURE_HYPERVISOR)) - static_branch_disable(&virt_spin_lock_key); + if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) + static_branch_enable(&virt_spin_lock_key); } =20 static void native_tlb_remove_table(struct mmu_gather *tlb, void *table) --=20 2.25.1