linux-kernel - Re: [PATCH v11 7/9] KVM: X86: Add userspace access interface for CET MSRs

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Message-ID: <08457f11-f0ac-ff4b-80b7-e5380624eca0@redhat.com>
Date:   Sat, 25 Apr 2020 17:31:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Weijiang <weijiang.yang@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, sean.j.christopherson@...el.com,
        jmattson@...gle.com
Cc:     yu.c.zhang@...ux.intel.com
Subject: Re: [PATCH v11 7/9] KVM: X86: Add userspace access interface for CET
 MSRs

On 26/03/20 09:18, Yang Weijiang wrote:
> There're two different places storing Guest CET states, states
> managed with XSAVES/XRSTORS, as restored/saved
> in previous patch, can be read/write directly from/to the MSRs.
> For those stored in VMCS fields, they're access via vmcs_read/
> vmcs_write.
> 
> To correctly read/write the CET MSRs, it's necessary to check
> whether the kernel FPU context switch happened and reload guest
> FPU context if needed.

I have one question here, it may be just a misunderstanding.

As I understand it, the PLx_SSP MSRs are only used when the current
privilege level changes; the processor has a hidden SSP register for the
current privilege level, and the SSP can be accessed via VMCS only.

These patches do not allow saving/restoring this hidden register.
However, this should be necessary in order to migrate the virtual
machine.  The simplest way to plumb this is through a KVM-specific MSR
in arch/x86/include/uapi/asm/kvm_para.h.  This MSR should only be
accessible to userspace, i.e. only if msr_info->host_initiated.

Testing CET in the state-test selftest is a bit hard because you have to
set up S_CET and the shadow stack, but it would be great to have a
separate test similar to tools/testing/selftests/x86_64/smm_test.  It's
not an absolute requirement for merging, but if you can put it on your
todo list it would be better.

Thanks,

Paolo