linux-kernel - Re: [PATCH v9 2/6] KVM: x86: Virtualize CR4.LAM

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Message-ID: <fbc522e315d261607869b1996adc05e3646e535e.camel@intel.com>
Date:   Wed, 7 Jun 2023 03:40:37 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>
CC:     "robert.hu@...ux.intel.com" <robert.hu@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "David.Laight@...LAB.COM" <David.Laight@...LAB.COM>
Subject: Re: [PATCH v9 2/6] KVM: x86: Virtualize CR4.LAM_SUP

On Tue, 2023-06-06 at 17:18 +0800, Binbin Wu wrote:
> Move CR4.LAM_SUP out of CR4_RESERVED_BITS and its reservation depends on vcpu
> supporting LAM feature or not. Leave the bit intercepted to avoid vmread every
> time when KVM fetches its value, with the expectation that guest won't toggle
> the bit frequently.

KVM only needs to do vmread once to cache guest's CR4, and presumable vmread is
a lot cheaper than a VMEXIT.  So I don't see the value of intercepting it if
there's no need to do.

But presumably I think we cannot allow guest to own this bit because KVM wants
to return a valid CR4 if LAM isn't exposed to guest?  Otherwise guest can still
set this bit even LAM isn't exposed to guest.

Am I missing something?

If not, your justification of intercepting this bit isn't correct and needs
update.