linux-kernel - Re: [PATCH v8 2/6] KVM: x86: Virtualize CR4.LAM

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Date:   Fri, 12 May 2023 10:49:36 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "robert.hu@...ux.intel.com" <robert.hu@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Gao, Chao" <chao.gao@...el.com>
Subject: Re: [PATCH v8 2/6] KVM: x86: Virtualize CR4.LAM_SUP


> > >   
> > LAM only applies to 64-bit linear address, which means LAM can only be enabled
> > when CPU is in 64-bit mode with either 4-level or 5-level paging enabled.
> > 
> > What's the hardware behaviour if we set CR4.LAM_SUP when CPU isn't in 64-bit
> > mode?  And how does VMENTRY check GUEST_CR4.LAM_SUP and 64-bit mode?
> > 
> > Looks they are not clear in the spec you pasted in the cover letter:
> > 
> > https://cdrdv2.intel.com/v1/dl/getContent/671368
> > 
> > Or I am missing something?
> Yes, it is not clearly described in LAM spec.
> Had some internal discussions and also did some tests in host,
> if the processor supports LAM, CR4.LAM_SUP is allowed to be set even 
> when cpu isn't in 64bit mode.
> 
> There was a statement in commit message of the last version, but I 
> missed it in this version. I'll add it back.
> "CR4.LAM_SUP is allowed to be set even not in 64-bit mode, but it will not
> take effect since LAM only applies to 64-bit linear address."

Yeah this does help.  Please add it back to the changelog.

> 
> Also, I will try to ask Intel guys if it's possible to update the document.
> 

Thanks.