Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] KVM: VMX: expose the host's ARCH_CAPABILITIES MSR to
 userspace
To:     =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        David Woodhouse <dwmw@amazon.co.uk>,
        KarimAllah Ahmed <karahmed@amazon.de>
References: <1519433546-33879-1-git-send-email-pbonzini@redhat.com>
 <20180226221300.GK22024@char.us.oracle.com> <20180301213205.GB29001@flask>
 <ccead1df-ba70-f365-c816-7892834a290c@redhat.com>
 <20180302214212.GB13606@flask>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <9cad32c8-fdb6-95db-2cd3-a0e3297b1811@redhat.com>
Date:   Wed, 7 Mar 2018 12:53:55 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.6.0
MIME-Version: 1.0
In-Reply-To: <20180302214212.GB13606@flask>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 02/03/2018 22:42, Radim Krčmář wrote:
> Ok, sounds good.  I've deferred it to rc5 as I think we'll want to use
> this to replace the auto setting:  I would not bet that it is going to
> be safe to expose future bits, so having the userspace always sanitize
> the capabilities would be safer (and more in line with what we do with
> other MSRs).  i.e. this patch would also
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 051dab74e4e9..86ea4a83af1f 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5740,9 +5740,6 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
>  		++vmx->nmsrs;
>  	}
>  
> -	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
> -		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, vmx->arch_capabilities);
> -
>  	vm_exit_controls_init(vmx, vmcs_config.vmexit_ctrl);
>  
>  	/* 22.2.1, 20.8.1 */

I don't know. There are good reasons for both behaviors, and especially
the following two for _not_ removing the rdmsr:

1) so far you could just pass the result of KVM_GET_SUPPORTED_CPUID to
KVM_SET_CPUID2, and expect the result to be "as close as possible to the
host";

2) having different behavior for VMX and ARCH_CAPABILITIES MSRs would be
confusing.

I think I'm leaning more towards the following direction: whitelist
ARCH_CAPABILITIES, like we do for the AMD LFENCE MSR already, and
default the AMD LFENCE MSR to the host value.

Thanks,

Paolo