linux-kernel - Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves

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Date:   Thu, 20 Jun 2019 16:55:05 +0800
From:   Xiaoyao Li <xiaoyao.li@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, Tao Xu <tao3.xu@...el.com>
Cc:     Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves_supported

On 6/20/2019 4:17 PM, Paolo Bonzini wrote:
> On 20/06/19 08:46, Xiaoyao Li wrote:
>>>
>>> It depends on whether or not processors support the 1-setting instead
>>> of “enable XSAVES/XRSTORS” is 1 in VM-exection control field. Anyway,
>>
>> Yes, whether this field exist or not depends on whether processors
>> support the 1-setting.
>>
>> But if "enable XSAVES/XRSTORS" is clear to 0, XSS_EXIT_BITMAP doesn't
>> work. I think in this case, there is no need to set this vmcs field?
> 
> vmx->secondary_exec_control can change; you are making the code more
> complex by relying on the value of the field at the point of vmx_vcpu_setup.
> 
At this point. Agreed. It's harmless to set a default value.

> I do _think_ your version is incorrect, because at this point CPUID has
> not been initialized yet and therefore
> vmx_compute_secondary_exec_control has not set SECONDARY_EXEC_XSAVES.

SECONDARY_EXEC_XSAVES is in the opt when setup_vmcs_config, and 
vmx_compute_secondary_exec_control() is to clear SECONDARY_EXEC_XSAVES 
based on guest cpuid.

> However I may be wrong because I didn't review the code very closely:
> the old code is obvious and so there is no point in changing it.

you mean this part about XSS_EXIT_BITMAP? how about the other part in 
vmx_set/get_msr() in this patch?

> Paolo
>