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Subject: Re: [PATCH v2 1/7] KVM:VMX: Define CET VMCS fields and bits
To:     Yang Weijiang <weijiang.yang@intel.com>
Cc:     rkrcmar@redhat.com, sean.j.christopherson@intel.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>
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>
 <20190129082917.GA32725@local-michael-cet-test.sh.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <e80bd3f5-7276-6c2a-3ad9-3ce0d0d0bff7@redhat.com>
Date:   Wed, 30 Jan 2019 09:32:59 +0100
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Sender: linux-kernel-owner@vger.kernel.org
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On 29/01/19 09:29, Yang Weijiang wrote:
> 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.
> Thanks, you're right, if we want to support live migration we need do that.
> However, this feature relies on quite a few external
> dependencies, e.g., cpu capabilities, kernel version and configuration, we
> don't support live migration right now. what's your opinion?

Live migration is generally a requirement for a feature to be accepted
in KVM.

Paolo

>> Thanks,
>>
>> Paolo
>>
>>>> In addition, PL1_SSP and PL2_SSP should be supported even if the guest
>>>> doesn't use them.  It makes sense to avoid intercepting them, but they
>>>> should still be supported and switched (possibly only if nonzero).
>>>>
>>>> Am I missing something, for example a dependency on host CET support?
>>>> If not, how was this series tested?
>>>>
>>> The guest CET feature is tested with kernel CET patches on internal
>>> virtual platform.
>>>