From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-yw1-f201.google.com (mail-yw1-f201.google.com [209.85.128.201]) (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 39F221BC06D for ; Wed, 31 Jul 2024 15:20:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722439205; cv=none; b=FAGB5GMNKTfnhtBkgPuJrxBY8zCQ5QbQZqQB7uhIvdcCZQth4SmklGkVexiwHPSIaHB9YkLaNv3+lEeABAqxh8rBDUnSxvKSvDOY+KVFGJuyl7VwZ3H2O4racG5tXmWtSW68phohvHDHldWMZSBioijy83fEYSl23XZwz8SAh/A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722439205; c=relaxed/simple; bh=mrnvQl6M+lb/gTQFQ85ejkCOiwiHss34vbITuDkqy5c=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=fYjxRwN+6tO+beA96I4si6TNNc5MJwodWaLAbcNZgz2CdbfiMriBUq25IYDWCxEg5xrMnoHpAwKepCcCK4aQFsnAML733bLKGqF7Z6SgcPbWwPx/RjhJIfmpUPODBk75DGCQ5JuNPWOysX0bWY5qI72Z8RITESpaujoPqZpbU2c= 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=FAfUbDq8; arc=none smtp.client-ip=209.85.128.201 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="FAfUbDq8" Received: by mail-yw1-f201.google.com with SMTP id 00721157ae682-650fccfd1dfso105493397b3.0 for ; Wed, 31 Jul 2024 08:20:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1722439203; x=1723044003; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:from:to:cc:subject:date:message-id :reply-to; bh=+awCnTQ6n6w0kReOKqFcxkIXj5QkErgyYJsc3T/RE3k=; b=FAfUbDq8Gb7FiRiYhZvQzWZTus2szjlVXl4ruAlQtmiq+6Yy+KMBeh2pSJx3I8uG5c Hg7cJYYOpSiW2GFqMKEO1Ro8XmFTZnUmImTg5bcVQ2nFeDTb4VWUHpjr1LH8BxpKJLhk I04lx5LHulGtsjPECWXbpLf9WagimgKewzu8q+qBCEkMKv1QZ79ApY4v4MSruwZO2xyi xnxewVstaGV2SzLwR5Q68JkvyAfgPlX2vc4tjz9XIuwIC/eb+IDgnzLJgPFyLBMfWVtE 45WqTaXI8SMHEvLxVialdg9rFo8jFCB8ezoGdsQJn1E+0bdUNH6ThPLhT1QTvmXgl6Ik CUXw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1722439203; x=1723044003; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=+awCnTQ6n6w0kReOKqFcxkIXj5QkErgyYJsc3T/RE3k=; b=lqYZsi3gCjgvjwWvtKsZ/NO40mZd0ykUJFJyoUYjv8w8Lw6U7ek6fj3I2EOBIEccbT Cyy7cyz4OlOgaT5lpUJsGTQWktx7FN3XtTs+2bHejUoC5pP7CEyy8zYIgHF3maMaQl1d 8B0BIBTwNU7JbgXSm2wQuL/hArS68hxwSX3DpDFfeeiaO5fu+EzPkR4FGB3yiGcN3Yuy jg3b8TY+HmZM6dD/1oQPHTA/7u25EgtI+g5rVIeHXobqqLbpegy6A9a7QWPeBDAEMSgf vTug6M/E8m11g1v6N/awjV+i44dFnm0SJBb/oU7pjvHOwuaZ7tiyjd+UR/95Kkou7g5D rEsA== X-Forwarded-Encrypted: i=1; AJvYcCUcqwpg4e8IyHaTroHcxlhsoOE+k3QefwkY1bfV+xLoq9518J8HqP8PA5ivkapjS7Dr1E78TiOnkUDSrphzkdE1W9Zn X-Gm-Message-State: AOJu0Yz/ikOdkDaX7jTvufNyS23/pHy/db24cxyKBvdYNL5Sek6DGz66 2WJ7D8rnpykLB2L3WRYZXwLZyDXW9Ujbq+IEcYTvkgSLdOMNRoBjIuqONECc64UHqEGg8PlVABA q7Q== X-Google-Smtp-Source: AGHT+IHkbvLVC/6tbuP4uZAhmbZgdzd7NoE0b5j4mmANdCAtR4G9YeAyrN7wTLF0fWbNpjsi5YL6he30vJc= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:690c:ece:b0:62f:f535:f2c with SMTP id 00721157ae682-67a053db18cmr4236867b3.2.1722439203063; Wed, 31 Jul 2024 08:20:03 -0700 (PDT) Date: Wed, 31 Jul 2024 08:20:01 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240730053215.33768-1-flyingpeng@tencent.com> <2e66f368-4502-4604-a98f-d8afb43413eb@redhat.com> Message-ID: Subject: Re: [PATCH] KVM: x86/mmu: Conditionally call kvm_zap_obsolete_pages From: Sean Christopherson To: Paolo Bonzini Cc: Hao Peng , kvm@vger.kernel.org, Peng Hao Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Wed, Jul 31, 2024, Paolo Bonzini wrote: > On Wed, Jul 31, 2024 at 1:19=E2=80=AFPM Hao Peng wrote: > > > So if anything you could check list_empty(&kvm->arch.active_mmu_pages= ) > > > before the loop of kvm_zap_obsolete_pages(), similar to what is done = in > > > kvm_mmu_zap_oldest_mmu_pages(). I doubt it can have any practical > > > benefit, though. > > > > I did some tests, when ept=3D0, kvm_zap_obsolete_pages was called 42 > > times, and only 17 times > > active_mmu_page list was not empty. When tdp_mmu was enabled, > > active_mmu_page list > > was always empty. >=20 > Did you also test with nested virtual machines running? >=20 > In any case, we're talking of a difference of about 100 instructions > at most, so it's irrelevant. It's not even remotely close to 100 instructions. It's not even 10 instruc= tions. It's 3 instructions, and maybe two uops? Modern compilers are smart enough to optimize usage of kvm_mmu_commit_zap_p= age() so that the caller inlines the list_empty(invalid_list) check, but the guts= of the zap code are non-inlined. So, as is, the generated code is: 0x00000000000599a7 <+55>: mov 0x8d40(%r12),%rbp 0x00000000000599af <+63>: cmp %rbp,%r15 0x00000000000599b2 <+66>: mov 0x8(%rbp),%rbx 0x00000000000599b6 <+70>: je 0x599d6 0x00000000000599d6 <+102>: mov 0x8d48(%r12),%rax 0x00000000000599de <+110>: cmp %r14,%rax 0x00000000000599e1 <+113>: je 0x59a5f 0x0000000000059a5f <+239>: mov 0x8(%rsp),%rax 0x0000000000059a64 <+244>: sub %gs:0x28,%rax 0x0000000000059a6d <+253>: jne 0x59a86 0x0000000000059a6f <+255>: add $0x10,%rsp 0x0000000000059a73 <+259>: pop %rbx 0x0000000000059a74 <+260>: pop %rbp 0x0000000000059a75 <+261>: pop %r12 0x0000000000059a77 <+263>: pop %r13 0x0000000000059a79 <+265>: pop %r14 0x0000000000059a7b <+267>: pop %r15 0x0000000000059a7d <+269>: ret and adding an extra list_empty(kvm->arch.active_mmu_pages) generates: 0x000000000005999a <+42>: mov 0x8d38(%rdi),%rax 0x00000000000599a1 <+49>: cmp %rax,%r15 0x00000000000599a4 <+52>: je 0x59a6f 0x0000000000059a6f <+255>: mov 0x8(%rsp),%rax 0x0000000000059a74 <+260>: sub %gs:0x28,%rax 0x0000000000059a7d <+269>: jne 0x59a96 0x0000000000059a7f <+271>: add $0x10,%rsp 0x0000000000059a83 <+275>: pop %rbx 0x0000000000059a84 <+276>: pop %rbp 0x0000000000059a85 <+277>: pop %r12 0x0000000000059a87 <+279>: pop %r13 0x0000000000059a89 <+281>: pop %r14 0x0000000000059a8b <+283>: pop %r15 0x0000000000059a8d <+285>: ret i.e. it elides the list_empty(invalid_list) check, that's it.