From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7465B3C078 for ; Thu, 26 Oct 2023 20:17:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="Xtlo0DJM" Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B1515129 for ; Thu, 26 Oct 2023 13:17:50 -0700 (PDT) Received: by mail-yb1-xb4a.google.com with SMTP id 3f1490d57ef6-d815354ea7fso962607276.1 for ; Thu, 26 Oct 2023 13:17:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1698351470; x=1698956270; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=k/Y1cqVei499MeRuO0JkL2AwtSOgPbZS08N4+Ku0vIU=; b=Xtlo0DJMwcg6S4xevIDOsvEqHIAD3oJWPu6TexObC/R5QGBs5RiNbzVIjTUXLr1SUH 6VWMaRfoV8+aSyNPJJc65D1qNOgYSqCFysaQMsGHEIiE4X4M2YQsB/Li+FuoWBb7TY5y o5I7j0/uvlA49lpn6ULMQwBv/g6PEQAPBQu0K5P1ywFHyOoqgIV980c8OSXUSbS7NwNQ P/pjGnoEjem1QgcCOFBMtQ1fnPo8GBrn+LYSxaJOsF8jx+dj7sUrl3igNlmQM9Kbz4oc OtXg2FqbktOKZBRPzpXMv4l7geQfUTEaUj9hUCt/hfqhW1oSkJVNscPCJaxcAOz6dQGI gNpg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698351470; x=1698956270; h=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=k/Y1cqVei499MeRuO0JkL2AwtSOgPbZS08N4+Ku0vIU=; b=M42wRtsgyxKPmrqPQhp/fBDRHhcqvTn+MCp6UaTLCThpsG/3DmOEyghlaAPslGq6jS ohacHbzKGJjn9R/fz4ir41MFy+pvWACFnImnXsLCmsjG9o+n+1DKGUSyy9UGLaxM7Mdh GP0UFkcEonsilHjJalKXeZ2halojY1Y5gnNzbhC+tZN/HMQ7GedqqRI6SGifmYG9P+L0 sTNOqFkCbSxwcAQ1ejBp/wsrQIAeOMYmFC96Vb7prr3ltyEzRiffz002XXNWy2BfbHsT EOccoQW9vDXwzIMi/6EqznRA7/NDXWuthrsp8bpCXek/j+eJieoVxn0Cw0W0F/5aDZR1 g/rw== X-Gm-Message-State: AOJu0YxGWxVAt9u8rBwfZxXX4NInUmkxjm09g2VQ+qVVYS7CHMaI8VFM iP5Ss4UnLrIyUWjsT59L+EpXdTZ+uwk= X-Google-Smtp-Source: AGHT+IFaYfepKEGrRhAf2VkMhklvq9ggBAMQ+KtkKZbuqh6XNf4Y8UxiI5C91vzIg6knFIOo4ZFLVR42pWM= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:6902:1788:b0:da0:c9a5:b529 with SMTP id ca8-20020a056902178800b00da0c9a5b529mr7678ybb.12.1698351469567; Thu, 26 Oct 2023 13:17:49 -0700 (PDT) Date: Thu, 26 Oct 2023 13:17:47 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20231025-delay-verw-v3-0-52663677ee35@linux.intel.com> <20231025-delay-verw-v3-6-52663677ee35@linux.intel.com> Message-ID: Subject: Re: [PATCH v3 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation From: Sean Christopherson To: Pawan Gupta Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Peter Zijlstra , Josh Poimboeuf , Andy Lutomirski , Jonathan Corbet , Paolo Bonzini , tony.luck@intel.com, ak@linux.intel.com, tim.c.chen@linux.intel.com, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kvm@vger.kernel.org, Alyssa Milburn , Daniel Sneddon , antonio.gomez.iglesias@linux.intel.com Content-Type: text/plain; charset="us-ascii" On Thu, Oct 26, 2023, Sean Christopherson wrote: > On Wed, Oct 25, 2023, Pawan Gupta wrote: > > vmx_disable_fb_clear(vmx); > > LOL, nice. IIUC, setting FB_CLEAR_DIS is mutually exclusive with doing a late > VERW, as KVM will never set FB_CLEAR_DIS if the CPU is susceptible to X86_BUG_MDS. > But the checks aren't identical, which makes this _look_ sketchy. > > Can you do something like this to ensure we don't accidentally neuter the late VERW? > > static void vmx_update_fb_clear_dis(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) > { > vmx->disable_fb_clear = (host_arch_capabilities & ARCH_CAP_FB_CLEAR_CTRL) && > !boot_cpu_has_bug(X86_BUG_MDS) && > !boot_cpu_has_bug(X86_BUG_TAA); > > if (vmx->disable_fb_clear && > WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))) > vmx->disable_fb_clear = false; > > ... > } Alternatively, and maybe even preferably, this would make it more obvious that the two are mutually exclusive and would also be a (very, very) small perf win when the mitigation is enabled. diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 0936516cb93b..592103df1754 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -7236,7 +7236,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu, kvm_arch_has_assigned_device(vcpu->kvm)) mds_clear_cpu_buffers(); - vmx_disable_fb_clear(vmx); + if (!cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF)) + vmx_disable_fb_clear(vmx); if (vcpu->arch.cr2 != native_read_cr2()) native_write_cr2(vcpu->arch.cr2); @@ -7249,7 +7250,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu, vmx->idt_vectoring_info = 0; - vmx_enable_fb_clear(vmx); + if (!cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF)) + vmx_enable_fb_clear(vmx); if (unlikely(vmx->fail)) { vmx->exit_reason.full = 0xdead;