From patchwork Fri Aug 2 20:01:36 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sean Christopherson X-Patchwork-Id: 13751947 Received: from mail-yw1-f202.google.com (mail-yw1-f202.google.com [209.85.128.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EA02B78C88 for ; Fri, 2 Aug 2024 20:01:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.202 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722628906; cv=none; b=VfP5sdIxzb28GCIFq0gPSq1+nXRsiSMdljn6A4nL0BqqH8Z+mnQI99afonmxjvC4bSCvIkX23Mu64U5itzaYRaHr1RzclbMJdow4lb3zje3Oq8+Z22Q0p+pYrvzE2TIjDsFkghar8iWFVjFCgel8XaHkv4ecvLeP4Q+C+BdZZac= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722628906; c=relaxed/simple; bh=YXcCG8Han6oPFZMVR5EJq3rsCuUTs8798czh5TGgT6g=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=J+T8DPfsUbUOzAC6z64Su1H2Erb4qVWegzUl86U2rTYbhwZij/nSKN3OsZZ1CO5d2ZBs27UeUBmtYeigR8+AnBaJGy29vQNnrXZGN+XGK0gBSD9o1Z/vAJL/yzdrNIelewfe8NWZLEzQtVDHU8EkeXhqRVO/aca2yPuwv8vhW9U= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=P/RgV1bf; arc=none smtp.client-ip=209.85.128.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="P/RgV1bf" Received: by mail-yw1-f202.google.com with SMTP id 00721157ae682-666010fb35cso59121477b3.0 for ; Fri, 02 Aug 2024 13:01:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1722628904; x=1723233704; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:from:to:cc:subject:date:message-id:reply-to; bh=ykfKcxdUa9fLrTb96clu+XLU5WSDBheRMe8+PcsjFjM=; b=P/RgV1bfBlyAg+RXdvMXskfAtdEZMK4thSDE6UJaVFge1JaA/2LiXvTAKVfO88IiMX vjrv6sMRdjT6ewChUEDo/WZvIi62A1A+E5rUkn9/s01Qqh5qMCb6SBrUPW8yr7V+s3uJ NJj8phrDNR7hrCWy0oIZ1yEQKyuUycNF4E1dVj6sDkoZYlO3dujPv/Rsn2BHbkyA+Cay cvDJtSYN9VZdKnx7J3xAEXWyvhAQsFzu7dTxuWqxLv3nwazJNogvMw/kOztfXibbY+8S l1In1bsFf4+Ow7KJ0euZbGcV5NHl7zx5zQJxdoSr1rpsDpuEIPwwn3q5FKzR85VgIRpl DSCw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1722628904; x=1723233704; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=ykfKcxdUa9fLrTb96clu+XLU5WSDBheRMe8+PcsjFjM=; b=l2xRWkz9er5GjCTwfASJRYPxDopxPfl8ryS6Died5zfV4Q0oelDSNkR+WNjRNq3h6R 9D5eBzLKx7gmJREOHEVoM8Lj7Ll9iP1xL0TTqrdNjcmR4l5Y4m22NVEOjjAeDaO6hoYK mrUNFurNjjl/X+qL+AxFIfdCB/42PinPxmoipG6yLWdN0vD1pCkn/EyJM2fLMUCk/TaI kJXbb1U96U7oa0BlBlLdXiOkP3rvVhw6GLe0+rckxDEdGUr2S5m9TCS4uGHgCKUia8at JA5s3Rxo3rCpCwv3iyItq4AIJvGRrOU6yPhKl5RgEpS0p8RXndHbOfOmLqszToICJb7S CyRw== X-Forwarded-Encrypted: i=1; AJvYcCUVf7yIKTBvk6+YVh7um7PLKNhJ4A4+/d8IyAAM1/VQ5a49+wHL40Nb/vmlXFQk7507OJ788aUcmswbN1JcNFrPNzR+ X-Gm-Message-State: AOJu0YwX55pG5iSqJ+448PoadobcEBE1T1bWqJcPbc+F74qf1SUS1GMj Hv5U4RgUx5nkLf4675PcBaKvzp9nKjAS83mvYHaaMTh1ONi9uTncndsEzjiSz3tOIbAf9IEu1PD I5Q== X-Google-Smtp-Source: AGHT+IGvtD2k+Z8XEDfN/3DgERyf+vS3FmJyCb7y6ktKFZouZ8vRLWbcTwmFsQsX8ntvC2Xbw1LZ/rv2S6Q= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:690c:2901:b0:644:c4d6:add0 with SMTP id 00721157ae682-6884f7ffdfcmr402097b3.1.1722628903604; Fri, 02 Aug 2024 13:01:43 -0700 (PDT) Reply-To: Sean Christopherson Date: Fri, 2 Aug 2024 13:01:36 -0700 In-Reply-To: <20240802200136.329973-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240802200136.329973-1-seanjc@google.com> X-Mailer: git-send-email 2.46.0.rc2.264.g509ed76dc8-goog Message-ID: <20240802200136.329973-3-seanjc@google.com> Subject: [PATCH 2/2] KVM: Protect vCPU's "last run PID" with rwlock, not RCU From: Sean Christopherson To: Marc Zyngier , Oliver Upton , Paolo Bonzini Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Sean Christopherson , Steve Rutherford To avoid jitter on KVM_RUN due to synchronize_rcu(), use a rwlock instead of RCU to protect vcpu->pid, a.k.a. the pid of the task last used to a vCPU. When userspace is doing M:N scheduling of tasks to vCPUs, e.g. to run SEV migration helper vCPUs during post-copy, the synchronize_rcu() needed to change the PID associated with the vCPU can stall for hundreds of milliseconds, which is problematic for latency sensitive post-copy operations. In the directed yield path, do not acquire the lock if it's contended, i.e. if the associated PID is changing, as that means the vCPU's task is already running. Reported-by: Steve Rutherford Signed-off-by: Sean Christopherson Reviewed-by: Steve Rutherford --- arch/arm64/include/asm/kvm_host.h | 2 +- include/linux/kvm_host.h | 3 ++- virt/kvm/kvm_main.c | 32 +++++++++++++++++-------------- 3 files changed, 21 insertions(+), 16 deletions(-) diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h index a33f5996ca9f..7199cb014806 100644 --- a/arch/arm64/include/asm/kvm_host.h +++ b/arch/arm64/include/asm/kvm_host.h @@ -1115,7 +1115,7 @@ int __kvm_arm_vcpu_set_events(struct kvm_vcpu *vcpu, void kvm_arm_halt_guest(struct kvm *kvm); void kvm_arm_resume_guest(struct kvm *kvm); -#define vcpu_has_run_once(vcpu) !!rcu_access_pointer((vcpu)->pid) +#define vcpu_has_run_once(vcpu) (!!READ_ONCE((vcpu)->pid)) #ifndef __KVM_NVHE_HYPERVISOR__ #define kvm_call_hyp_nvhe(f, ...) \ diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 689e8be873a7..d6f4e8b2b44c 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -342,7 +342,8 @@ struct kvm_vcpu { #ifndef __KVM_HAVE_ARCH_WQP struct rcuwait wait; #endif - struct pid __rcu *pid; + struct pid *pid; + rwlock_t pid_lock; int sigset_active; sigset_t sigset; unsigned int halt_poll_ns; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 91048a7ad3be..fabffd85fa34 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -486,6 +486,7 @@ static void kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id) vcpu->kvm = kvm; vcpu->vcpu_id = id; vcpu->pid = NULL; + rwlock_init(&vcpu->pid_lock); #ifndef __KVM_HAVE_ARCH_WQP rcuwait_init(&vcpu->wait); #endif @@ -513,7 +514,7 @@ static void kvm_vcpu_destroy(struct kvm_vcpu *vcpu) * the vcpu->pid pointer, and at destruction time all file descriptors * are already gone. */ - put_pid(rcu_dereference_protected(vcpu->pid, 1)); + put_pid(vcpu->pid); free_page((unsigned long)vcpu->run); kmem_cache_free(kvm_vcpu_cache, vcpu); @@ -3930,15 +3931,17 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_kick); int kvm_vcpu_yield_to(struct kvm_vcpu *target) { - struct pid *pid; struct task_struct *task = NULL; int ret; - rcu_read_lock(); - pid = rcu_dereference(target->pid); - if (pid) - task = get_pid_task(pid, PIDTYPE_PID); - rcu_read_unlock(); + if (!read_trylock(&target->pid_lock)) + return 0; + + if (target->pid) + task = get_pid_task(target->pid, PIDTYPE_PID); + + read_unlock(&target->pid_lock); + if (!task) return 0; ret = yield_to(task, 1); @@ -4178,9 +4181,9 @@ static int vcpu_get_pid(void *data, u64 *val) { struct kvm_vcpu *vcpu = data; - rcu_read_lock(); - *val = pid_nr(rcu_dereference(vcpu->pid)); - rcu_read_unlock(); + read_lock(&vcpu->pid_lock); + *val = pid_nr(vcpu->pid); + read_unlock(&vcpu->pid_lock); return 0; } @@ -4466,7 +4469,7 @@ static long kvm_vcpu_ioctl(struct file *filp, r = -EINVAL; if (arg) goto out; - oldpid = rcu_access_pointer(vcpu->pid); + oldpid = vcpu->pid; if (unlikely(oldpid != task_pid(current))) { /* The thread running this VCPU changed. */ struct pid *newpid; @@ -4476,9 +4479,10 @@ static long kvm_vcpu_ioctl(struct file *filp, break; newpid = get_task_pid(current, PIDTYPE_PID); - rcu_assign_pointer(vcpu->pid, newpid); - if (oldpid) - synchronize_rcu(); + write_lock(&vcpu->pid_lock); + vcpu->pid = newpid; + write_unlock(&vcpu->pid_lock); + put_pid(oldpid); } vcpu->wants_to_run = !READ_ONCE(vcpu->run->immediate_exit__unsafe);