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;
Date:   Mon, 4 Mar 2019 10:56:21 -0800
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Yang Weijiang <weijiang.yang@intel.com>, rkrcmar@redhat.com,
        jmattson@google.com, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, mst@redhat.com, yu-cheng.yu@intel.com,
        yi.z.zhang@intel.com, hjl.tools@gmail.com,
        Zhang Yi Z <yi.z.zhang@linux.intel.com>
Subject: Re: [PATCH v2 1/7] KVM:VMX: Define CET VMCS fields and bits
Message-ID: <20190304185621.GG17120@linux.intel.com>
References: <20190122205909.24165-1-weijiang.yang@intel.com>
 <20190122205909.24165-2-weijiang.yang@intel.com>
 <5a31eef0-2b6c-bdc2-c54b-5dc4cecf084d@redhat.com>
 <20190128103345.GA29895@local-michael-cet-test.sh.intel.com>
 <4d9a5393-163b-f32a-4657-cec54093ee88@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <4d9a5393-163b-f32a-4657-cec54093ee88@redhat.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Jan 29, 2019 at 04:19:34PM +0100, Paolo Bonzini wrote:
> On 28/01/19 11:33, Yang Weijiang wrote:
> >> There is no code in this series to pass these fields to and from
> >> userspace, and also to save/restore U_CET, INT_SSP_TAB, PL0_SSP and
> >> PL3_SSP across context switches.
> >>
> > The kernel consumes these MSRs, please see kernel CET patch:
> > https://lkml.org/lkml/fancy/2018/11/20/225
> 
> Still, even if the kernel saves these fields across context switch in
> XSAVE areas, KVM must support accesses to the MSRs from userspace, for
> example in order to perform live migration.
> 
> For example, when reading/writing these in kvm_set_msr or
> kvm_get_msr_common, you would have to do a read/write from the host
> MSRs.  You also have to put kvm_load_guest_fpu/kvm_put_guest_fpu calls
> in __msr_io.

Paolo, can you elaborate on why KVM would read the host MSRs?  Wouldn't
kvm_{get,set}_msr() pull the values from the VMCS when necessary?