linux-kernel - Re: [PATCH 01/15] KVM: VMX: Use x86 core API to access to fs_base and inactive gs

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Message-ID: <1c851214-2873-69c0-0ba6-d82374c26722@linux.alibaba.com>
Date:   Mon, 22 Nov 2021 11:27:15 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 01/15] KVM: VMX: Use x86 core API to access to fs_base and
 inactive gs_base



On 2021/11/21 23:17, Thomas Gleixner wrote:
> Lai,
> 
> On Thu, Nov 18 2021 at 19:08, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>>
>> And they use FSGSBASE instructions when enabled.
> 
> That's really not a proper explanation for adding yet more exports.
> 

Hello

---
When a vCPU thread is rescheduled, 1 rdmsr and 2 wrmsr are called for
MSR_KERNEL_GS_BASE.

In scheduler, the core kernel uses x86_gsbase_[read|write]_cpu_inactive()
to accelerate the access to inactive GSBASE, but when the scheduler calls
in the preemption notifier in kvm, {rd|wr}msr(MSR_KERNEL_GS_BASE) is used.

To make the way of how kvm access to inactive GSBASE consistent with the
scheduler, kvm is changed to use x86 core API to access to fs_base and
inactive gs_base.  And they use FSGSBASE instructions when enabled.

It would add 2 more exports, but it doesn't export any extra software nor
hardware resources since the resources can be access via {rd|wr}msr.
---

Not so persuasive.  If it needs to be accelerated in the preemption notifier,
there are some other more aggressive ways.

Thanks
Lai